OK, I restored my phones with twrp running cyanogenos and lineage, also some others. The TWRP allway backed up a *.zip file. Here are some files in the backup folder.
I didn't run a restore already on Mi8...
What did you exactly backup?
Found on TWRP: "The default backup options in TWRP are system, data, and boot. For most cases, this is sufficient for backing up a ROM.
On most devices shipping with Android 6.0 or higher, the device maker has enabled dm-verity as a security measure."