Some Apple Watch Series 5 and Series 6 SE owners experienced issues with the watch not charging after entering the low-power reserve. According to user forums, it is a far wider issue.
First, the good news – if you are experiencing issues Apple Watch Series 5 or SE, the global service announcement acknowledges the fault. The remedy is a mail-in repair free-of-charge, however. However, Apple wants you to upgrade to the latest watchOS 7.3.1 first to see if this fixes the issue.