I've done some more research on this and have the following to add:
And on a Dell workstation.
An analysis of fwupd's history database indicates that one was attached for a few minutes five years ago. On both affected computers.
I now believe this is because I didn't "service fwupd restart" after the removal. Once I did that, the history was indeed reset.
It seems the proper way to clear the history is to run "fwupdtool clear-history". I *think* one doesn't need to restart the fwupd service when doing things this way.
One can probably also selectively purge the Dell dock-related records from the history, at least if one is willing to edit the history database directly with sqlite3. Or maybe just clear the "needs activation" flag with an SQL UPDATE.
Red herring, it turns out. No regular expressions involved, just ancient and long-forgotten events.
I'll let this bug report stand as documentation in case someone else runs into something similar. The user interface could be friendlier but that's a generic wishlist item.