This is a stupid and unnecessary discussion. Back in the 1990s Oceanic made an early dive computer that allowed you to set an oxygen percentage, rather than just using air. Out of an abundance of caution, they forced the percentage to jump to 50% O2 after a dive as a way to try and ensure you would set the damn thing before every dive. It was a noble but bad idea which they backed off from in their very next version of that computer -- more than two decades ago -- and gave the diver the choice of keeping that dumb auto reversion to 50% or turning it off. The manual was clear as a bell how do do that, and anybody who read the manual, or talked to a friend or to a dive shop, turned it off.
So the OP comes on and starts this thread that rants about how his ancient computer keeps reverting to 50% O2 and he can't turn it off. OK, he's just lucky that ancient computer is still working!
This is NOT a problem today. Computer either do not revert to 50%, or you can turn it off.
There is no need for getting into design principles.....it is irrelevant to the thread. The thread was ill-posed to begin with, and the problem was solved over two decades ago.
In conclusion, six angels can dance on the head of a pin.