Cannot add merge request dependency

I tried adding a merge request as a dependency to another MR in the same repository. I tried:

• pasting the MR url
• writing !3 in the field
• writing company/project/!3 in the field

After each of these and pressing enter a "ticket" appears in the "Merge request dependencies" field. But after I click "Save changes" it's gone. No error message is displayed. It won't appear in the MR overview page.

It's strange because it's already worked once in this project. I had a MR with dependency from the same repo. I couldn't add another dependency, though.