After migrating an older Drupal 6 site with 20,000 media items to Drupal 8, I found a strange problem with image uploads. On the Drupal site, using Image FUpload and Adobe Flash, I could upload up to 99 images in one go. On the new Drupal 8 site, I was only able to upload 20 images, even though I didn't see an error message or any other indication that the rest of the images I had selected through the Media Image upload form were not successfully added.
I could choose 21, 40, or 500 images, but only 20 were ever added to an album at any time.
There were no apparent warnings on the screen, so I just assumed there was some random bug in the Media Image Entity or Media module suite that limited uploads to 20 files at a time.
But due to an unrelated error, I glanced at the PHP logs one day, and noticed the following error message: