Sometimes MR's and their issue forks become outdated and you need to reset them to the latest commit of the origin branch.
But take care, that will discard all changes on this issue forks branch!
Ok, you're sure?
Here you go:
git reset --hard origin/1.x
git push --force
Assuming the branch you'd like to reset your issue fork branch on is 1.x. Typically you want to reset it to the Merge Requests (MRs) target branch!