You can, in fact, compose a concatenation directly in the “pure expression” type box. Again, this may seem fiddly, but reading my explanation of how to do this (and why you have to) might be illuminating. Do not expect Bubble to “fix” this as nothing is in fact broken. The assignment field in a custom state is simply a general-purpose expression field not a text-specific expression field.
Read on and I think you’ll understand what I mean (you will also learn how to get to a full-featured text expression field from any general expression field, using a boolean expression):