To reproduce:
Window 1
Open a PR (note that there are no merge warnings)

Window 2
Login as an admin and create a new default reviewers rule that applies to the PR you just opened

Window 1
Try to merge the PR
You should see the merge error dialog show up
Click the "View details" link
You will see the "how to resolve a merge conflict" error modal rather than any information around the required reviewer.

Update: For non dry-run vetoes, this is the only way that the veto is shown (it isn't shown on the disabled merge button because it is calculated on merge, rather than canMerge). The error message needs to contain details about the veto that occurred.