@Mr.C Exactly this, its annoyingly random
I did the phone to phone transfer though a few days ago with no issues, wear the watch throughout the process, it takes abit longer and you can't use the phone while its completing but I had no issues so will be doing this again
It prompts you at some point if you want to use the watch with the new phone (at this point im guessing its paired with both devices) and then it just carries on with the transfer. Take note my achievements didn't transfer over for another day but then they just appeared
I never unpaired my watch once from the old phone, the transfer finished and then I just did the usual turn off find my and sign out of iCloud to get the phone ready to send off to MM