I am having the same problem with Apple Watch 6 and iPhone 12 ProMax.
I use a cable to connect to Car Play on a Volvo.
After the first occurrence I called Apple Support. They were absolutely no help. Their "solution" was to unpair the watch & then pair it again. That didn't work.
After it happened again today, I re-started the phone and the issue resolved. The call appears on my watch and my MacBook.
This was not an issue with my Apple Watch 4 & my iPhone 8 Plus using a cable to connect to Car Play on the Volvo.
Bleueyz
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