Apple Watch fails to connect after transferring an old iPhone to a new iPhone 15

Okay I grabbed my old phone out of the closet and went into the Watch app and hit Unpair. It spun its wheels for a few minutes and then said "Error during unpairing process." Then it said something about doing a factory reset, which I did. Then I tried again to set it up on my new phone, which worked eventually, but only after I tried using the "New watch" button instead of "Existing watch," which doesn't make a lot of sense, but then nothing about this process did.. Oh well, anyway, it's set up now, so all good 👍

I don't know what it's doing behind the scenes during the migration process. The prompt was "Do you want to transfer your Apple Watch to your new iPhone?" and I clicked yes. I would assume that after I clicked "Yes" it would do whatever needed to be done to make that happen, but maybe not?

I certainly didn't manually unpair it, but then again, I didn't manually unpair anything. Yet for the most part, all my data and settings and so forth were moved to the new phone. Is there a bug in the iPhone migration process that affects certain Apple Watches? If there is, I couldn't find anything online about it.