I experience the exact same issue...
I mistakenly did "fastboot boot twrp.img" and now my phone is in a bootloop, it shows the Mi logo once (no dots) and then again and boots back into recovery (TWRP or Mi-Recovery 3.0, whatever is on your /recovery partition)
Only way to boot the phone is by doing "fastboot boot twrp.img" and after a little while it starts booting, but without working audio (due most likely kernel differences).
flash_all_except_storage.sh on latest china stable (which I came from) no fix, the issues persist....
Doing reboot --> System in TWRP gives the same loop, fastboot continue does it too.
If anyone finds a solution let me know, I'm searching for it as well.....
EDIT: Flashing xiaomi.eu latest dev (MIUI10.3) made the dots appear in the boot process but never finished booting (too many boot issues in logcat), flashing back the .12 china stable and booting with the "fastboot boot twrp.img" does no longer boot the system..... I think I messed up my data now as well
EDIT2: Wiped full system, its booting again... So sad all my data is gone, need to reinstall everything again >: (