I ran into the problem multiple times while making IMBC (Italian meringue buttercream). I only made it successfully once sadly.
I manage to get as far as whipping the egg whites with the sugar water and making them rise successfully. It's glossy, and looks like what it's suppose to look like. But when I add my room temperature butter, while whipping it at high speed, it immeadietly deflates. I've attempted the recipe by Cake It (a YouTuber), many times and it never works. I tried other recipes as well, but as soon as that butter mixes in, it turns into a liquid mess.
Some factors that might change my results vs everyone else:
I live in a high altitude area.
It was during the summer and my house does not have ac.
The butter was "too" room temperature?