SuttaCentral
In SN 16.10 and SN 16.11, I came across this narrative twice：
Cavittha ca pana thullatissā bhikkhunī brahmacariyamhāti.
But the nun Thullatissā fell from the spiritual life.
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Obviously, this phrase does not mean “disrobing”, because Thullananda cannot disrobe herself twice, so what exactly does it mean?
I believe the commentary explains (for the first instance cited) that she returned to lay life, switched to white clothing.
It may well be possible however that both suttas refer to the same event.
Yeah, I read these last sentences as a like “where are they now” kind of thing, like you used to see at the end of movies.
This is very similar to the suttas which report an interlocutor as having attained enlightenment after a discourse:
Then that mendicant, living alone, withdrawn, diligent, keen, and resolute, soon realized the supreme end of the spiritual path in this very life. He lived having achieved with his own insight the goal for which gentlemen rightly go forth from the lay life to homelessness.
He understood: “Rebirth is ended; the spiritual journey has been completed; what had to be done has been done; there is no return to any state of existence.” And that mendicant became one of the perfected.
~ e.g. SN 22.63
These suttas don’t report exactly how long “soon” means: these “post-scripts” may well have occured months or years after the main events of the sutta. How much more so in these two suttas, where we don’t even have a “soon” to guide us.
I think the disrobing of Thullananda was an important event for the early Buddhist community. Such an understated and simple description strikes me as hard to understand. Are there any other suttas mentioning the disrobing of Thullananda?
Why was the disrobing an important event for the early Buddhist community?
There’s a lot about her in the Vinaya, but not about her giving up the training.
It you haven’t already seen it, this is what the Dictionary of Pali Proper Names has on her:
Thullanandā
A nun, one of four sisters who all joined the Order, the others being Nandā, Nandavatī, and Sundarī-Nandā.
Thullanandā appears to have had charge of a large company of nuns, all of whom followed her in various malpractices (Vin.iv.211.239, 240, 280).
Thullanandā was well-versed in the Doctrine and was a clever teacher. Pasenadi, king of Kosala, is mentioned as having come on two occasions to hear her teach, and was so pleased with her eloquence that he allowed her to persuade him to give her the costly upper garments he was wearing (Vin.iv.254‑256).
She was greedy for possessions, and was later accused of misappropriating gifts intended for other nuns (Vin.iv.245‑246, 258).
She was fond of the company of men, and frequented streets and cross-roads unattended that she might not be hindered in her intrigues with men (Vin.iv.270, 273).
She seems to have regarded with sympathy women who succumbed to temptation and to have tried to shield them from discovery (Vin.iv.216, 225, 230 f).
She bribed dancers and singers to sing her praises. She could brook no rival, and especially disliked Bhaddā, whom she deliberately annoyed on more than one occasion (Vin.iv.283, 285, 287, 290, 292).
She was fractious and would wish for something, but when that was procured for her, would say it was something else she really wanted (Vin.iv.248, 250).
She was evidently an admirer of Ānanda, and was greatly offended on hearing that Mahā-Kassapa had called Ānanda “boy,” and gave vent to her displeasure at what she considered Kassapa’s presumption. However, we are told that soon after that she left the Order (S.ii.219 ff ).
She befriended Ariṭṭha when he was cast out of the Order (Vin.iv.218). The Suvaṇṇahaṃsa Jātaka was related in reference to her, and she is identified with the brahmin’s wife of the story (J.i.474 f).
In the early Buddhist community, Thullananda was one of the most famous monastics. Her behaviors always led the Buddha to formulate rules, but technically, she had never violated any rule. Finally she disrobed herself. Isn’t it confusing that Buddhist texts say almost nothing about her disrobing?
It seems you have raised a meaningful issue about the Bhikkhuni in the early Buddhist community. This issue possibly can be a good research topic in early Buddhism studies.
These are two different nuns, Thullanandā and Thullatissā. Chinese sources identify Tissā as Nandā’s sister.
I don’t think it is about disrobe.
Even if she still wears the robe, there was no possibility for her to live the pure life as Brahma (aka attain any awakening) due to wrong view and bad deeds against noble, Ven. Maha Kassapa.
As many Sutta said:
These dear beings did bad things by way of body, speech, and mind. They spoke ill of the noble ones; they had wrong view; and they chose to act out of that wrong view. When their body breaks up, after death, they’re reborn in a place of loss, a bad place, the underworld, hell.
There are many example that people still wear robe, but never get into any awakening, but went to hells such as Devadatta, Kokalika, etc.
So be careful with your actions. Don’t block your own path.
Indeed Bhante 
they do seem similar in character but they don’t have quite the same name
Thullatissā is mentioned nowhere else in the Pāli Canon and might be seen as a misspelling of Thullanandā, especially given the similarity of their names and behaviors.
Tissā regularly appears in Chinese sources.
All schools, including the Pāli school, agree that many of Thullanandā’s family members joined the sangha.
Tissā was not a member of Thullanandā’s family.
In the origin-story for Bhikkhuni Parajika 1, we are told that Thullanandā and her three sisters (Nandā, Nandavatī, Sundarīnandā) had become nuns together. Sundarīnandā engaged in sexual intercourse with a layman and disrobed later.
There is no Tissā in the Pali Canon that matches the bad nun Thullatissā in SN 16.10.