Unable to merge MR when dependencies are resolved

Summary

Unable to merge MR when all cross-project dependencies have already been merged.

Steps to reproduce

I did try to reproduce, but the proof of concept is here gitlab-com/www-gitlab-com!29986 (merged).

Example Project

gitlab-com/www-gitlab-com!29986 (merged)

What is the current bug behavior?

The option to merge is disabled.

What is the expected correct behavior?

I should have the option to merge the MR

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Possible fixes