Hi Nicky - I’ve seen quite a few posts mentioning the same thing which has occurred since one of the later releases/patches - for example:
https://forums.zwift.com/t/zwift-wont-sync-with-garmin/222469?u=dean
My suggestion is to log this with Zwift support - support.zwift.com as I dont believe I have seen them acknowledge this (might be wrong on that).