This bug is still present in self-hosted 3.1.3 on Debian. It’s cosmetic rather than functional: Monica shows the default category in the dropdown when editing but doesn't actually change it to that. It remembers the original category.

I think there is potentially a functional aspect to this (or perhaps a related issue). When creating a new activity, if I want to select the default category, I need to select another category and then select the default one again. Otherwise no category is recorded/saved. So the default category displayed is not selected as a default category when creating a new activity. This is happening for me on a self-hosted instance, version 3.1.3.

Still present in 3.2.0 self-hosted on Debian.