The issue is that the C memory model allows more behaviours than the memory model of x86-64 processors. You can thus write code which is incorrect according to the C language specification but will happen to work on x86-64 processors. Moving to arm64 (with its weaker memory model than x86-64) will then reveal the latent bug in your program.

OpenBSD famously keeps a lot of esoteric platforms around, because running the same code on multiple architectures reveal a lot of bugs. At least that was one of the arguments previously.

The compiler relies on the language and programmer to enforce and follow a memory consistency model