Let’s say you throw in a protean Meowscarada and then have it use triple axel, which will turn the mons typing to ice. Next turn you Terrastalize to Stellar. Will you’re type be locked as Ice or will it revert back to Grass/Dark?
A Stellar Terastallization will retain Protean's type change. This means, in your example, that Meowscarada will still have the defensive properties of the Ice type. I tested this on Showdown; you can find the replay link here.
It will be the same type as the move you used with Tera it won’t have anything special it just becomes that type during ur turn
