I have the same issue - a 13mini, a 14, and an AW.
An Apple support guy told me that when you unpair an AW from an iPhone, an automatic backup will be made.
I'm not so sure about that.
Sometimes, when re-pairing my AW with the other phone, only (sometimes weeks!) older backups are being presented to chose from.
I hav not yet found a way to reliably force a watch backup to be made.
Once I thought it will only backup if something has changed on the AW - but no. I intentionally changed something on the AW, then put them both on the charger next each other, 1m apart from the Wifi router, left them there over night, and no new backup was made.