"Delete source branch" on merge requests can't be ticked (regression)

Summary

The 'delete source branch' tick box on merge requests can no longer be ticked on gitlab.com

I don't recall seeing this before so it's probably a regression in 11.9.

Steps to reproduce

On an open MR, try ticking 'delete source branch' button.

Example Project

https://gitlab.com/fintechlabs/fapi-conformance-suite/merge_requests/432/diffs

What is the current bug behavior?

The button can't be ticked. Same result in Safari & Chrome.

What is the expected correct behavior?

Should be tickable.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com