Apple Watch restarts on rowing exercise then battery goes to 10%

81% is only 1% above the threshold for deteriorated battery health. Your battery is definitely a contributing factor. Probably way more than an update. Although to be fair, the series 4 is also very likely at the end of its update life (meaning it will likely not be compatible with the next major watchOS release later this year since it will be 6 years since its original release in September). Not to say it can’t keep being used of course but your battery is definitely knocking on the door to battery replacement time. Just something to keep in mind. Do you need a new watch…that’s a personal choice but it might be time to start thinking about it. Not sure if there are any suggestions that can help with what you’re experiencing but you can report it to Apple in case it is a bug with watchOS. The battery health though if you took it to an Apple Store or authorized repair facility would likely be the first thing they would notice and mention it will be time to address sooner than later.

If you still have the original battery it very well could be the problem. What is the battery health percentage in settings? If it’s less than 80% then yes, your battery is indeed the problem.

I have this same problem. The watch is fully charged, then out of nowhere, it dies to 10%. Not a slow drain. In 1 second, it says "I'm dying"... I have to force reset the watch multiple times to get it to realize, oh yea, I do have power... its' really frustrating.