Source branch not removed after MR

When a MR has finished with the selected squash commits and delete source branch. The source branch does not delete. There is a spinning circle indicating that something is happening.
Using Firefox Network Inspector I found a couple errors.

Source map error: Error: JSON.parse: unexpected character at line 1 column 1 of the JSON data
Resource URL:
Source Map URL:
Uncaught (in promise) Error: SIMPLE_POLL_TIMEOUT

If you refresh the page the MR is completed and you have an option to delete source branch. When selecting from this screen the source branch does delete.