Kafka Streams skipped-records-rate sensor producing nonzero values even when FailOnInvalidTimestamp is used as extractor

According to the code and the documentation for this metric, the only reason for a skipped record is an invalid timestamp, except that a) I am reading from a topic that is populated solely by Kafka Connect and b) I am using `FailOnInvalidTimestamp` as the timestamp extractor.

Either I'm missing something in the documentation (i.e. another reason for skipped records) or there is a bug in the code that calculates this metric.