blocked merge having closed dependencies

Summary

It is not possible to merge a MR that depends on a closed MR.

Steps to reproduce

1. Create a MR (a) that depends on a closed MR (b)
2. Try to merge MR a from Gitlab's UI

What is the current bug behavior?

Although all dependencies are merged or closed, Merge is blocked with the following inconsistent message: "Merge blocked: all merge request dependencies must be merged or closed."

What is the expected correct behavior?

Merge should be allowed.