You can only merge once the items above are resolved. - till I add comment

Hello all, I spot this problem, occasionally after MR is opened the Merge button is greyed out with following error: You can only merge once the items above are resolved. And there are no obvious reason for that. Nothing in the log, no discussion and no CI configured.

Workaround is to add one comment to the MR, and Merge button is now green.

Version is Gitlab CE 12.8.6