It is not always the strap - at least in my case. I bought a new strap the first time these strange readings started happening because people on the Watchuseek forum suggested it and replacing the strap did nothing, I still had spontaneous bad readings, not every move, but totally random. Some FW updates were OK, others I had the problem … the pattern was usually every other update was bad for HR.
Now I have two straps and two smart sensors at home and changing either just the strap or both sensor and strap doesn’t help.
I don’t care much about HR anymore, it is just a guideline when I’m cycling uphill so I don’t overdo it, otherwise my legs give out sooner than my heart and lungs do. 🙂 It is just something that bugs me and should be working OK-ish.