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Deleting a branch after merging it results in "there may be uncommitted changes"

Hello, I'm trying to understand Git and the mess I've made. Some time
ago, I did crazy things like adding to master even though I was
working in develop, leaving it a commit ahead and X commits behind. I
did crazier things, like trying to amend a previous post's message.

Anyway, I follow a very simple merging model for this one-person
project. Recently, I made a new local branch off of develop called
feature-printing. After checking out feature-printing, making my
changes, and committing changes, I merged it with develop. I then
immediately tried to delete feature-printing, which resulted in a
prompt asking if I was sure since it might contain uncommitted
changes. Though I've seen this problem many times on the internet, I
haven't seen it in the context of literally just merging. There are 0
steps between merging and deleting the old branch.

All of this is done in Visual Studio's GUI for Git. Any ideas?