Terapagos-Tera doesn't revert to Baby on faint, but Terapagos-Stellar does. Ogerpon-Tera reverts
Explanation from a comment that I left below:
Terapagos-Tera is the one that the baby turns into after Tera Shift activates, which is shown toward the start of the video. You can see Tera Shift doesn't need to activate again because it stayed in the Terapagos-Tera form the whole time while it was fainted/revived.
Stellar is the one that activates after Terapagos-Tera uses its Terastallization (confusing, I know). The test for it begins at 2:03, where you can see after it faints and is revived it becomes the baby again and Tera Shift has to activate when it switches in.
The baby form is unplayable in battle, the video only seeks to answer the question of when Terapagos has to transform again with Tera Shift when it switches back in after fainting.
Done both to contribute to the battle mechanics research effort at Smogon and for my own personal curiosity. These videos are made public since Indigo Disk in case anyone stumbles into them and finds them helpful or informative. All the videos can be found in the playlist below, including ones not visible directly on the channel.
https://www.smogon.com/forums/threads...
• Pokemon SV Mechanics Testing and Research