That's surprising, and it doesn't seem to be new to v18.
In the second case when you're hitting return, what's happening (incorrectly) is an alias expansion - it will always replace the text with whatever you have defined as the comment expansion in Languages -> C/C++ -> Comments -> Doc Comments/Edit Expansion. Which just wipes out what you typed.
So I'll fix that for the beta do it does not do the alias expansion then. Arguably, it should be smart enough to do something more like the comment editor in that case, but that's too big of a feature change to sneak into the beta.