When mask-agnostic and tail-agnostic mode is set, QEMU's default behaviour is to still act like the mode is set to tail-undisturbed and mask-undisturbed, so a programmer could forget to set the mode correctly when using masks or differing vector sizes, and CI will still pass in some cases.
These QEMU_CPU flags toggle the behaviour that -agnostic modes should be treated as if they always destroy data, so it will more likely detect these problems, although QEMU might not respect this fully for all operations.
VideoLAN code repository instance