The usual answer by all the usual Apple fan-boys and -girls is that it must be that the battery is at the end of its life cycle.
I am pretty certain this is not the case: When it's almost fully charged I can use my iPad for at least 6-8 hours before it indicates that it is less than 10% charged. If I let it charge to fully 100 percent, then I can now be sure that it will need at least one reboot before I can use it, but usually two.
I can see that this has been a problem for many people for several years. Apple's answers never mention this as a software problem, nor as a recognized and fixed software problem, nor do they explicitly state that it's a hardware problem.
If it's indeed a hardware problem, then you wonder why the device software can't create a diagnosis error message instead of just shutting down, or at least once it boots up again.
So I conclude that they won't fix this bug because it would extend the lifetime of many iPads for years, because most people who can afford it, are quickly ready to buy a new, fancier, iPad.