I have noticed same problem with fit2 pro. With phone is ok only then battery is optimized. If you are using any battery power saving - gps signal won't be accuratre. Today I run only using fit2 pro and distance measured was 4.3km but real distance is around 5.5km
I noticed that too in the latter days of my Gear Fit2 Pro. I say "latter days" because I have abandoned it. It got wet and after a LOT of fanagling with Samsung, they repaired it. It seemed to work ok for awhile but then stopped with the same message I got when I first had it repaired. It was out of warranty so I took it apart and there was a fair amount of blue copper oxide visible in the circuitry. The screws are covered by inadequate rubber-ish plugs that IMHO, can't help but leak. I decided I had had enough and shelved it; too many problems.
Interestingly, my GearS3 has similar, occaisonal GPS and heart-rate cut-outs. Even my Note9 has this problem. This is definitely a Samsung issue so I'm pretty skeptical about their quality. I love the GearS3 and still use it but, of course, it is not waterproof and can't be used for swimming. When it works (99% of the time), it is very accurate.
I have gotten my money's worth out of the GearS3 and I love the Note9 but the Gear Fit2 Pro is not a good device.
FWIW, I just ordered a Huawei GT2. It is not as feature-full as the GearS3 (not quite!) but it has great battery life and is good for swimming (and is on sale right now!).