A couple of days ago, Amazon began distributing new Kindle Software for the 11th and 12th generations and all versions of the Kindle Scribe. It looks like Amazon has stopped pushing out firmware 5.19.3 and all Kindles are now on the older 15.12.2. Amazon has not disclosed the reason, but many Kindle users have reported that 5.19.3 caused battery drain, with a fully charged Kindle completely out of juice within a couple of hours. Page-turn speed was greatly diminished, and the UI was very sluggish.
If you have already installed 5.19.3 automatically from Amazon, or if you installed it manually, there is nothing you can do. It is impossible to roll back an updated Kindle or Kindle Scribe to a previous firmware version, even if you factory reset the device.
Sadly, Amazon has been releasing more software updates full of bugs over the past year. Many users reported that most updates are being written by AI rather than by dedicated Amazon engineers. A very good social media thread said as much, it is a good read.it.
Hopefully, Amazon can sort out the buggy mess the Kindle software has become. Some useful features in the software were pulled, such as dictionaries on all Kindles, which look better and more polished. Sideloaded manga files have been broken for a few months, but they are now working again with slimmer margins.