Some replies in Comments in MRs arent being saved

Summary

When posting a reply to an existing comment in an MR, the comment is shortly shown, then again removed, and never actually persisted.

Steps to reproduce

Open MR in a project in gitlab-ce 9.2.6, have 1 user comment in the code, and then try to reply to that comment with another user. Doesnt work every time though, and we could not quite figure out what exactly reproduces it.

Example Project

We could not reproduce on gitlab.com. This might be due to the difference between gitlab-ce and gitlab enterprise.

Relevant logs and/or screenshots

Screencapture of the problem:

This is an image from the POST request REQUEST PAYLOAD made when replying:

This is an image from the POST request RESPONSE made when replying: