I can replicate.
If left empty, the “title is required” and “post can’t be empty” warnings visually bounce, and the topic stays in the composer.
If it’s only the “You must choose a category” warning, then the topic gets ‘sent’ and an error modal pops up (“Category can’t be blank”)
I’m not sure if it’s a bug, or it just handles this validation differently?
I think it is possible to change the placeholders in the composer, though you may need to write up a bit of custom code to add to your theme. I’m afraid it’s out of my skill level, but you may get more attention if you start a new #support topic asking about that specifically.