I'm a bit lost in this thread, but I've written up what I know for other dummies like me

Aapt2 is an x86_64 standalone binary used to build android APKs for various CPU targets

Previous versions of it used a simpler instruction set, but the new version requires an extra SIMD instruction SSE4. A lot of CPUs after 2008 support this, but not F-droid's current server farm?

> Our machines run older server grade CPUs

So a bit of both of older hardware, and not-matched-with-consumer-featureset hardware. I'd imagine some server hardware vendors supported SSE4 way earlier than most, and some probably supported it way later than most too.