When trying to initiate the auto trigger based on a variance intensity metric, we will get the original metric from the variance intensity metric since one has to exist. If for any that the metric is not available, then the error comes up.
It seems there is possibility that metric is covered inside the log. In this example a FrontEnd, can be shut-off though. So if it is shutoff while tracking it, these messages can pop up until it expires which is by default 15 minutes.
Ideally, this message should not be there in the logs and may be tracked in supportability. This message has no performance impact and is only seen in DEBUG/VERBOSE logging in APM 10.5.1 or later.