That's expected. Devices with
A/B partition scheme don't have a recovery partition. You can only boot in recovery mode with
fastboot boot twrp.img. But to permanently flash recovery to device which can be booted into by proper key combination, you need to unpack both of your
boot.imgs (A/B) from device and replace the
ramdisk with the one extracted from
twrp.img. Then repack and flash
boot.imgs back to device.
On TWRP this has been previously achieved by flashing a special ZIP file, or one could do manually. Now the official TWRP release includes the option
Install Recovery Ramdisk:
Use magiskboot to allow repacking boot images for installing TWRP
recovery.img had two major components; kernel and ramdisk. On A/B devices, no ramdisk is required for normal boot because
system.img is mounted as
rootfs when processing Device Tree (
dtb) which is appended to kernel image usually. So the
boot.img is that of recovery. Since kernel is same for a device (other than build configurations), so only ramdisk needs to be replaced.