Follow this, it worked for me
Many devices will replace your custom recovery automatically during first boot. To prevent this, use Google to find the proper key combo to enter recovery. After typing fastboot reboot, hold the key combo and boot to TWRP. Once TWRP is booted, TWRP will patch the stock ROM to prevent the stock ROM from replacing TWRP. If you don't follow this step, you will have to repeat the install.
Or after flashing recovery do not reboot and instead use this command
fastboot boot recovery.img
Then you will have temporary TWRP. From there again flash TWRP. Both these process worked for me.