I just read this now from one of the most important researchers in the recent history of Burma. Please excuse me if this passage, or this research, is already known to you. I know that there has been a lot of research and examination of the matter of Bhikkhuni ordination, including the historical dimension of it. However just in case you didn’t already know about this, i have to say that Professor Than Tun (d. 2005) was one of the most eminent researchers in Burma, with his PhD done at London University, and long time visiting professor in Japan and USA. The text below suggests that he knew much more than just this passage, regarding this matter, and I am tempted to investigate if he had written about the historical presence of Bhikkhunis in Burma in more detail. Here is the quote:
There is another important point in the Buddhism during the period under survey [AD1000-1300]. That is the presence of ‘bikkhuni’ in the Order then. According to tradition there have been no female ascetics in the Buddhist Order since AD 456. But we find epigraphic evidence assuring their presence even in the 13th century. I am sorry that I do not gave time to go into some details of this very important point. The following are a few popular names of the ‘bikkhuni’: Phun-mlat-so Uiw Chi Taw, Skhin Uiw Kram Pan San, Uin Chi Up Ni, Skhin Brahmacari, Phlun-mlat-so Uih Tan San, and Skhin Uiw Pam.
_p. 44. Essays on the History and Buddhism of Burma. by Professor Than Tun. Kiscadale Publications. 1988.