The merge from the issue branch was a fast-forward merge, i.e. a merge
that only advances the branch pointer of the target branch (the one you
have checked out) and doesn't produce any new commits.
Unless you add additional options to the push Gerrit refuses to accept
changes pushed to refs/for/branchname if the commit SHA-1 is known, i.e. it
isn't a new commit. In your case the contents of the issue branch had been
pushed to Gerrit before.
To force the creation of a merge commit pass
git merge. That merge commit will end up as a reviewable
change in Gerrit.