can't consistently use 03-mini-high. why the heck not?

I have a pro account and I can start out using o3-mini-high on a new chat, but it will eventually switch to 4o at some point and not let me switch back (within that chat).

It also won't currently let me to use o3-mini-high within a project today, but it did yesterday.

Does anyone have any idea what is going on here? I'm trying to use 4o and it is failing very badly on code that o3-mini-high wrote perfectly for me yesterday. Now I'm stuck :/