yes I have been on pie since beta 1, now I am on beta 4, cheers.
i had a need to restore my nandroid on pie. and i failed with above method mate. device was rebooting in fastboot mode after restore and twrp lost when i restarted with above steps
I was stuck on black screen with notification light on.\
i tried to adb fastboot boot twrp blu got this error
>fastboot boot twrp.img
downloading 'boot.img'...
OKAY [ 0.630s]
booting...
FAILED (remote: Failed to load/authenticate boot image: Load Error)
finished. total time: 0.651s
then i tried this command
fastboot flash boot twrp-3.2.3-x_blu_spark_v9.86_op6.img
target reported max download size of 536870912 bytes
sending 'boot_a' (28788 KB)...
OKAY [ 0.666s]
writing 'boot_a'...
OKAY [ 0.116s]
finished. total time: 0.803s
then also flashed to partition b
fastboot flash boot_b twrp-3.2.3-x_blu_spark_v9.86_op6.img
target reported max download size of 536870912 bytes
sending 'boot_b' (28788 KB)...
OKAY [ 0.669s]
writing 'boot_b'...
OKAY [ 0.148s]
finished. total time: 0.841s
tried to fastboot boot twrp blu spark but failed
then tried to fastboot official twrp that worked
restored my backup which was saved on phone and thankfully sdcard didnt get messed maybe because of being decrypted
but still was not able to boot into system
then i adb pushed OOS Pie Stable zip to phone
flashed OOS Pie without any wipes as previous backup was restored.
flashed twrp Blu spark and tried to boot into system
but still returned back to twrp but now on blu spark
flashed magisk 17.1
restarted system
and voila backup has been restored on OOS PIE.
it had password and fingerprint even that made no issue