Worker / Metrics / Subrequests - seems mistake in subrequest counter

My worker gets invoked by cron and makes 20 fetch requests to 20 different URLs. The Metrics view shows only 19 requests (18 uncached and 1 cached). The log does not show any errors, all 20 fetches are logged as normally done.

Any idea?

P.S. So far, my assumption that this is a bug: somewhere the last_item_id is used instead of .length().