The requirements to avoid the anti-rollback hazard are explored here (answered) Install /e/ on FP5 with newer Security Patches.
You probably read similar. It could be the subject of Report an issue if eosinstaller did not warn you to note the Android SPL of the installed version at time of unlock.
Was the phone recently updated?
If yes can you recognise for certain your build from https://support.fairphone.com/hc/en-us/articles/18896094650513-How-to-manually-install-Android-on-your-Fairphone ? The Android SPL appears in each build details.
The method to ensure safe locking is covered here [HOWTO] a Safe Guide to install "IMG-e-builds" even if the /e/ web installer fails.