[lidavidm]

https://www.ralfj.de/blog/2019/07/14/uninit.html perhaps (the OP also talks about this when linking to a talk about jemalloc)

[MrBuddyCasino]

Interesting.

Tl;dr: its not to do with any hardware concept, the compiler can substitute any value for a read of uninitialised memory, and the value does not have to be stable.