[lgg]

We have no idea if it was even undefined in the first place, since the PPU documentation is not public.

You are correct thought, often guarantees about behavior can be made after the fact. Generally what happens these days is someone determines they need or want to use a certain undefined behavior. The hardware people then go, look at the RTL for the chip and confirm it behaves in a certain way, and then they UPDATE the documentation to explicitly document the newly guaranteed behavior.

Conversely, often something that is documented to work just doesn't. Especially in the case of console developers who tend to be using early steppings of custom silicon. In that case sometimes when you go to the HW team they fix it in the next stepping, sometimes they document it as errata and move on.