Somehow, I solved it, but I'm not completely sure how. <sigh>
It MIGHT had to do that I had confused smbpasswd -x with smbpasswd -d (delete vs. disabled), that I was trying to delete the user with smbpasswd -d instead that using -x.
Everything got fixed up while trying to compare two logs, one for the user that was working and one for the user that wasn't working, so I tried to reset everything: removed both users from /etc/passwd and from the samba user database with smbpasswd -x and added them again.
I also brought the service up and down several times just to ensure changes were being applied.
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