Can't boot into TWRP recovery with Oxygen OS 5.1.3 using fastboot (stock recovery on device)

[tim-k]

After installing Oxygen OS 5.1.3 (everything stock, device unlocked), I can no longer boot into TWRP recovery using fastboot. I am just trying to boot, not flash, TWRP.

This is how I tried to boot into TWRP:

adb reboot bootloader

fastboot boot twrp-3.2.1-2-dumpling.img

That just goes to stock fastboot.

Try again with:

fastboot boot twrp-3.2.1-2-dumpling.img

The following is returned:

downloading 'boot.img'...
OKAY [ 0.577s]
booting...
FAILED (remote: Failed to load/authenticate boot image: 00000050)
finished. total time: 0.614s

I am assuming that this is due to the following item from the 5.1.3 changelog:

Updated bootloader for system security enhancement

BTW, this is the output from fastboot oem device-info:

(bootloader) Verity mode: enforcing
(bootloader) Device unlocked: true
(bootloader) Device critical unlocked: false
(bootloader) Charger screen enabled: true
(bootloader) enable_dm_verity: true
(bootloader) have_console: false
(bootloader) selinux_type: SELINUX_TYPE_INVALID
(bootloader) boot_mode: NORMAL_MODE
(bootloader) kmemleak_detect: false
(bootloader) force_training: 0
(bootloader) mount_tempfs: 0

[ghost]

Any update?