Pageviews = 0 on a segment

My Matomo installation has 2 segments:

  • Mobile (device=smartphone)
  • Desktop (decide=desktop/tablet)

The “All visits” graphic shows correctly the total of pageviews for a certain day (the sum mobile+desktop is a reliable number), while one segment (Mobile or Desktop) sometimes shows 0 pageviews on the graphic.
I followed the official FAQ, that recommends to invalidate the report for the day and the segment, and then wait for a new archive cron to rebuild the correct report data. Both invalidation and archiviation are successful, but the segment always keeps showing 0 pageviews.
I tried to invalidate the segment through CLI and API, I tried auto archive and also manual archive, but nothing happened… all steps end successfully, but the pageviews for the specified segment are always 0 in the pageview graphic.

I followed the following FAQs:

Did any of you experience the same problem? Can any of you suggest a different solution?
Thanks in advance!