This is part of the
cell reset bug. The current work around is to only use power armor FRAMES that you BUY. Not the ones that you find out in the world. Those are essentially containers that get 'reset' when the cell reloads, randomizing whatever the contents were (i.e., Super Duper Flame Painted T-60 parts) into basic rusty T-45 parts.
For what I gather, using console commands to set yourself as the 'owner' of a found frame may prevent this (FYI, the command is
setownership and IIRC the player's ID is
14). But better safe than sorry: KELO sells frames at Goodneighbor (just don't forget to take it home ;).
Another stop-gap is for it to always be worn when you save, either you or your current companion (they will exit it at some point, should you ever dismiss them, so keep that in mind).
I can't recall if this bug is present in the vanilla game, or if this happens to me because I've got mods installed. However, I can confirm that using purchased frames will keep you safe. On the plus side, you've got a few frames sitting around that will spawn free fusion cores for you.