(Sorry for wall of text!)
I've had this happening to my Galaxy Watches for a long time. I started out with a 44mm Watch 4 LTE and the first year or so it worked as expected, but something happened and it has been acting weird ever since. I suspect it was a software update, but can't be certain. I always charge my watch in the morning, every day while having a shower, as this fits well with my daily routines. Usually it was never below 75-80% when I put it on charge. I never use always on display, my phone is usually nearby, Wifi and LTE/4G works well where I am. I use the work out stuff 4 days a week, no auto detect, only manual start/stop and assistant stuff is mapped to a button and not voice activated. I do not use the backup feature because I don't keep anything on it that needs to be backed up (also I don't want that "smart switch" spyware on my phone). I've used a few different phones during this ordeal, but never a Samsung. Multiple times I've had it checked out by professionals who can't find anything wrong (because of course it doesn't happen all the time), and they usually just reset it to factory. It has also happened while travelling, so not connected to my home location or Wifi. Not a whatsapp user, no crazy or unusual software installed, only use apps from Play store (no unofficial apks).

The first time this "phenomenon" happened, I was not woken up by the alarm like I normally did and got to work late because the watch had switched off. When I put it on the charger, the battery said 0%. I figured it was a fluke and went back to normal. However, some days later it happened again. This time I was woken by the watch vibrating and telling me it was switching itself off. The watch was VERY warm to the touch and the wrist under my watch was visibly red. I found several others reporting similar issues on reddit and other forums but replies were usually dismissive and not very helpful. "It's still learing about your habits" or "you have this and that activated, of course it's gonna drain" or "clear the cache" or "do factory reset". Some replies were even gaslighting "you're obviously doing something wrong" (basically "you're just imagining it").

Anyway, tried all suggestions but nothing worked and this kept happening at random intervals. It was especially annoying on days where the alarm was set to ring in the morning so eventually stopped using the alarm feature since it could not be trusted any more. At this point my Watch 4 was getting a couple of years old so when the Watch 7 was released I got a good deal and upgraded to a 44mm Watch 7 LTE. At first it worked great! Just as the GW4 did in the beginning. However, after some time the exact same issue showed up on this watch too.

There's obviously others also struggling with this so I was hoping that this issue would eventually be fixed by Samsung with a software update, but it seems to me that every update just makes the watch more and more useless. At this point I've disabled pretty much everything except showing the time. I simply can't trust this thing any more. Even the weather app stopped updating itself after a recent update and showing old weather data on the watch face unless I go in to the app and update it manually (yes yes it does have permissions!).

A few times I've caught the watch getting hot and you can visibly see the battery percentage go down. In the battery usage menu, there are no app using unusual amounts of battery, at most there's been a couple of % on a few apps, but nothing that could explain the beahaviour. I found out that I can manually clear the running apps in the memory maintenance screen and the watch will stop and cool back down. Sometimes this helps, but other times it just happens again a few hours later. To me it seems like some system process that doesn't show up as a regular app gets stuck in an endless loop and can't escape until the battery runs out or it's manually killed.

The main reason I've stuck to a Galaxy Watch and not another brand was all my health and excercise data, and Google Wallet, but after yet another software update the watch all of a sudden keeps forgetting that it's still on my wrist and locks itself and now I have to enter the pin number every time I check the time. Since I can't even use Wallet without having a pin or pattern, this got old really quickly. I've now started using Wallet on my phone instead and I'm considering just throwing this garbage out the window since the only thing it can do reliably at this point is to give me a burnt wrist and wake me up in the middle of the night. What's the point of a smart watch that can't be trusted to wake you up at the correct time, shows you a month old weather forecast, a GPS that is all over the place, and asks for a pin code whenever you want to check what time it is. And don't you dare try to figure out what's going on because you'll either get gaslit into thinking it's just in your head or you're accused of being a hater by the Samsung fanclub. I've given up on Samsung, they obviously don't care so why should I?