Every time I swim in the pool, the HR sensor gives me unreal values (90/100 bpm) for the first 15/20 minutes, and then suddenly starts giving real values (140/150 bpm) until the end of the workout.
My old fenix 6 also did this same behavior, so I don't think it's a broken sensor problem (I would be really unlucky to have found two).
The watch is worn correctly (tight enough not to move and 1 cm above the bone).
Did the watch measure HR well for you right from the start? Do you have any advice to ensure that I always read the HR correctly in the water?