If you have TWRP PIN problem, check your file system type of /data partition, in TWRP choose <Wipe> ==> <Advanced>, select target partition (say 'data'), then press <Repair or Change File System>, now the file system type will show on screen, then you can just press back to leave.
In my case, my /data partition type was 'emmc' when TWRP asked me for password, I changed it to 'f2fs' manually which solved the TWRP password issue. But after I reboot the system (beta 8) it changed back to 'emmc', I don't know why but I believe somehow beta8 changed it back to 'emmc'.
In my case, my /data partition type was 'emmc' when TWRP asked me for password, I changed it to 'f2fs' manually which solved the TWRP password issue. But after I reboot the system (beta 8) it changed back to 'emmc', I don't know why but I believe somehow beta8 changed it back to 'emmc'.