Currently PTL's is_cray() method looks for "alps_client" in momlogs to detect if it is a cray or not.
But If on a system mom has been running for more than a day continuously then the log message that PTL expects is present in the logfile of the day pbs_mom daemon started.
PTL on the other hand check for log message in the current mom_log file and then fails to find it and returns "is_cray()" as false.
It seems wrong to assume that PTL is run in the same day when daemons come up.
workaround is to restart the daemon for which you are calling log_match (in this case it is mom)