How exactly did you install it? Did you use the App or did you flash it manually?
Two ideas:
- Volume Up button got stuck … not likely because you can now boot Fairphone OS, it would boot into the stock Android recovery everytime now with the button being stuck.
- TWRP was installed into the wrong partition, if that’s even possible (don’t know). Did you exactly follow the “Fastboot Install Method” at twrp.me?
You can boot TWRP without flashing it and then wipe your whole phone clear and install TWRP and the OS from scratch.