Not a fix but a workaround: use Low Power Mode (this is as opposed to Power Reserve). You will need to be quick given a limited window before the power runs out but from Control Center press the battery percentage. You can activate Lower Power mode: it makes the battery percentage indicator yellow. You should do so for the fixed time period (up to three days) because otherwise Low Power mode deactivates itself at 80% and you will be back in the same cycle. Low Power mode works, and will allow the watch to gain sufficient charge that it is usable — though power drain is still much more than normal and I would recommend also enabling Theater Mode and disabling always-on to keep the screen off. The watch faces are buggy and tend to crash and restart, but otherwise it seems functional, to the extent Low Power Mode isn't reducing functionality by design. The relevance is that this provides the route for applying a patch or B3 as and when, as you can get the watch above the 50% threshold, and escape the power drain cycle. And, for those who care, with some management it may be possible to keep some Fitness streaks going.