Fitness app appears to redouble counting steps

I wear a pedometer watch (not Apple). When I check my step count on the fitness app I get a reading which appears sensible. If I then cause my watch to sync (with its native app) it also gives an understandable reading (they are not identical and I do not expect them to be) However if I now return to the fitness app my step count has vastly increased - not just by a few step but by thousands. It is almost as if the watch count has been added the fitness total. A recent set of numbers were Fitness 3300, watch 2930, fitness after syncing watch 6555. My phone is an iPhone 12 running iOS 18.0.1. The watch is a VSTAY Band-485.