Unable to add multiple assignees to PR until created

When I create a PR in our private repo, I cannot add multiple assignees until after having created the PR. While creating it, I can only add one assignee.

Trying to add multiple assignees while creating a PR, will result in the second checkmark next to one’s name in the list appearing like normal, but that name will not be added to the list of assignees. It’s like it’s pretending to add a secondary assignee, but not actually doing it.

It should work identically to adding multiple tags, which does work normally.