Apple Watch OS has released a minor bug fix update. Newer version 7.3.1 indicates a charging error regarding the power supply and is only available for Series 5 and SE. .
In particular, the individual Series 5 and SE models cannot move to the charging process after using the power reserve. Apple recently had a thing in this regard Support-Document Published. Apple has not yet provided additional information, such as security-related updates.
WatchOS 7.3.1 Fixes an issue that prevented some Apple Watch Series 5 and Apple Watch SE devices from charging after using Power Reserve.
Update the process
You can download and install the 64MB update directly from Watch under Settings> General> Software Update. Alternatively, you can do this via the Watch app on the paired iPhone using the same menu path. In both cases it is necessary to be on the watch charging buck after downloading and installing the update. Installation begins when the battery is at least 50% charged.
They are: apple.com