I noticed difference issue on my activities too, for me duration differs by couple of seconds from sum of HR zones, but I considered it comes from HR zone times aren’t displayed with 1/10th seconds precision.
But your case is way strange. 🤔

Update: now that I look more carefully on my yesterdays run, when the watch lost connection with the strap at the end of the run, the difference is more then 10 seconds. Which is strange, if you consider that from precision, only max. 5 seconds may come (if there are 5 zones).