That’s stock recovery.
Did you boot into recovery immediately after the fastboot installation, or did you do a normal boot first? If the first boot after flashing TWRP is a normal boot, it often gets overwritten by the stock recovery. If the first boot is directly into recovery, TWRP fill take measures to avoid that from happening, as mentioned on the TWRP install page:
Note 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.
Anyway, as you’ve found out, installing Fairphone open also gets you TWRP, so you should be good to go.
EDIT: Apologies if this a double post - it seems the forum is behaving a bit strangely for me, as I’m now seeing posts with inconsistent time ordering as well as experiencing long loading times.