Today I stumbled on the answer go into apple health select your profile go to apps and in this case revoke Veryfit app write permissions for all dara, after that i went into veryfit app and it told me to go to apple health and enable veryfit app write permissions. nearly instantly the two apps relinked and synced data for the first time in a week.
While I can't be 100% sure this removal of the write permission fully and then re enabling it may be a common solution to data sharing issues it seems to work for phone to watch notifications too.
I think apple health basically did a sanity check on the incoming data, decided it wasn't valid and disabled that data point. while keeping the sane data. so there you go try completely revoking write permissions in apple health for the app and then them enabling them. it might have been an app update that triggered the fault, as i had updated the app on the date it last gave good values.