Steps to reproduce

It's a tricky one, as it seems to be intermittent/flakey. 

1. Ensure Squash is enabled in Project Settings -> Merge strategies
2. Create a pull request and click Merge button in the top right
3. In the merge dialog, select Squash as the merge strategy
4. Type a custom commit message
5. Click Merge button

It might happen for other merge strategies too, but with 'squash' I was able to replicate this intermittently.

Expected results

 Pull request is merged

 Commits are squashed together

 Custom commit message is applied

Actual results

 Pull request is merged

 Commits are squashed together

 Custom commit message is applied