After I edit someone’s post on desktop I always have the “Add Edit Reason” if I want to add a reason for my edit. I usually do so, but I usually moderate on mobile and don’t have that option.
Repro:
Make sure you are on the mobile version and, as a moderator, just click on the edit pencil to edit someone’s post. You will not have the option to add an edit reason.
Yes, I can do that and do, do that. A problem with that is I can’t see what I’m typing then. That makes it a pain when I type the wrong thing, because then I have to go back and re-edit just to add the reason again.