NPSP Contact Merge with External Ids

Feel like I am missing something obvious

On the the NPSP Contact Merge screen, I am trying to merge two contacts. In this org, I have added an External Id that is marked as unique, and in the merge, I want the External Id on the loser to be the one that is kept after the merge. But I get an error saying I have a duplicate External Id (which suggests there is an update happening before the old record is deleted)

But if I use the standard UI Merge Screen, it works fine, so the NPSP version is obviously working differently from the SF version.

So can I use the NPSP version if I have external Ids in my org? Or do I need to tell folks to just use the regular UI?