The famous DN 11 Kevaddha sutta is bedevilled by uncertainty in how to spell the title, which is the name of the protagonist. Alternatives include Kevaddha, Kevaṭṭa, Kevatta, and Kevaḍḍha. Most scholars in the English-speaking world use Kevaddha, though not, so far as I can see, for any particular reason. The MS text which SC uses has Kevaṭṭa.
The Chinese parallel at DA 24, however, is helpful. I’m not sure if this has been noted before, so I thought I’d put it here. The name in Chinese is 堅固. Now, sometimes names in Chinese transliterate the Indic, and sometimes they translate it. Given that the sound of these words (jian gu) isn’t anything like Kevaddha, and that the two words are more or less synonyms, it seems they are a translation.
But what are they translating? Well, each term, and the idiom as a whole, means “solid”. The DDB conveniently lists several Sanskrit terms that this may stand for. Among these are dṛḍha, dhruva, dṛḍhatara, dṛḍhatā, dṛḍhatva, dṛḍha-parākrama, dṛḍha-sāra, etc. While far from definitive, this does suggest that the Chinese was based on a text that used the vocalized and aspirated form dh rather than ṭ.
And it might even suggest a preference for the retroflex ḍ rather than the dental d. This is, I think, less certain, as the dental is preferred in many of the Prakrits (DA was likely translated from something Prakritish rather than classical Sanskrit.)
In any case, I think the Chinese text does support the spelling Kevaddha.