I struggled with this for a while and think I have found a solution.
I'm new to git, so forgive me if I use the wrong names for things.
You may have a problem using the --squash when running the add git command.
Try to remove the subtree (delete the remote repository and delete all local files, commit and click). Then repeat adding the subtree without the --squash option.
Then I jumped into my subtree repository, made some changes, committed and clicked, then jumped back to my main superproject repository and made the git tag. This gave me some error related to the fact that the working tree has modifications. To get around this, I did a git validation wizard, then hit git, and then tried the subtree again. It worked.
Hope this helps.
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