branch is not deleted during merge request if "delete source branch" is checked

Steps to reproduce

• Create merge request
• tick "delete source branch"
• make some load on gitlab instance (i.e. consequently accept several MR-s and launch corresponding CI jobs)
• accept MR in question

What is the current bug behavior?

• merge request accepted and merged
• 
• branch is not deleted
• if page is refreshed

What is the expected correct behavior?

Branch should be automatically deleted

it's Gitlab CE 11.8.2 (gitlab-ce@b1268e93775a992c990a8d378491b19c59463545) here. Omnibus docker, official image. And it was OK in prior versions.

I believe, it was OK in 11.7.x as I haven't accepted MRs in 11.8 yet. So I have updated from 11.8.0 to 11.8.2 today and had to accept some MRs. And stumbled upon it.

I've also had another issue with git api (gitaly) responsiveness omnibus-gitlab#3984 (closed) which I'm not sure yet resolved too.