Then from twrp retrieve the content of /proc/last_kmsg and /sys/fs/pstore
Tried it again with v215 on my Xiaomi Mix 2S and I hadn't remembered it quite right, the phone actually goes: MI boot logo, black, MI boot logo, fastboot. I hold the volume up button when it turns black but it still goes to fastboot.
I can reboot into TWRP and check these locations you gave, but:
last_kmsg doesn't exist,
pstore folder is empty,
and for good measure last_mcrash contains "none".