It seems that the FastPing check routine does not look at the “overwrite ip” address of a host.
If a host has a hostname which is unreachable and therefore needs the overwrite IP function to work, this results in the host going UP/DOWN every couple of minutes. The regular poller would check by IP and see the host as online. And after a minute the FastPing check will declare it dead because the hostname does not resolve.
fetchDevices used by the fast ping: https://github.com/librenms/librenms/blob/fa024e3e4e537b9d6c742b634b077d763960d45c/app/Jobs/PingCheck.php#L158
pollerTarget function used by the poller to select the address to use: https://github.com/librenms/librenms/blob/fa024e3e4e537b9d6c742b634b077d763960d45c/app/Models/Device.php#L46