Bugs in consumer electronics are almost a reality, especially in smart devices, but many of them can happily be fixed with a software update. Sadly, some bugs often strike some hardware parts, which then need some correction or replacement. The Apple watch series 5 and Apple watch SE seem to be like that, which unexpectedly refused to charge while entering power reserve mode. Fortunately, when these symptoms occur, Apple offers to fix the smartwatches free of charge.
Power Reserve is like the low power mode of the Apple Watch where it retains the battery life and shows you the time. It does it by disconnecting Apple Watch from their combined iPhone contact and restricting the access to other features.
Unfortunately, some owners of these watches seemed not to be able to charge their Apple Watches after entering the mode, as a useful function otherwise has caused headaches that Apple is now trying to alleviate. This undesirable behaviour, the support document provides no clarification however, one thing is clear: an authorised technician must fix the devices.