Since release 8.9.13 all 256/8GB XA variant became XE due to Xiaomi policy as it is made for china market only. It is the same with official MIUI releases of course.
It also means you can't re-lock the bootloader anymore using Global ROM otherwise you will get bricked
Installing persist was not needed for me.
Follow this in order to avoid issues on Mix2S changeover to Pie:
1. Install latest TWRP 3.2.3-0 V8 (you can find it on XDA)
2. Boot to recovery
3. Wipe all: cache, dalvik/ART, data, system
4. Format Data
5. Reboot to TWRP
6. Copy ROM.zip to internal...
Please open following bug report, log in with your Mi account and click on big orange coloured rectangle shaped CONFIRM button:
It is bug report regarding Mi Mix 2S unstable ultrasound proximity sensor operation during phone calls