Android Dynamic System Updates (DSU) Broken ONLY on Xiaomi.eu (Previously Working on Stock)

Device is unlocked and with Xiaomi.eu installed to primary slot. Attempted to use Dynamic System Updates for sideloading a test system.img known to boot with EU base (test images include stock image and a GSI image) when installed via fastbootd, but the newly installed DSU system appears to NOT pass AVB (even though AVB should have theoretically been disabled?).

This behavior is NOT observed on stock Xiaomi ROMs (neither official Global nor CN) - on official ROMs the alternative DSU system...

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