I have this problem. It used to happen once in a while, and rebooting the phone would fix it. After updating to 14.7.1, which purportedly fixes the problem with a TouchID phone, my FaceID phone would not unlock the watch. Rebooting did not help. I finally dove in and did the unpair/repair dance, which fixed it for about 2 days. But it is once again broken. I am not very willing to keep unpairing and repairing to keep this working, so really hope that is not the solution going forward. It's painful to re-add all my credit cards to Apple Pay after unpair/repair. Especially as one of my cards requires a call to the issuing bank.
I think an obscure bug with FaceID was introduced when Apple fixed the TouchID problem. But evidently it's an obscure one or else many more people would be complaining about this.
Any ideas from anyone would be welcome. I may go ahead and do the unpair deal again, but I'm not eager to dive into that if there is any other way.