I’m using custom image styles inside of Drupal 8 for the first time (I’ve always just forced admin users to upload various image sizes) – but for ease of use for admins I’ve incorporated that feature on a couple of sites.
It seems like the images are expiring after a certain period of time (I don’t know the exact timeframe since it’s now just being reported to me). For example the sized images are referenced in views – but don’t appear. When you go to the page node, however where the generated image is force re-created, then it appears within the view again.
Is there a setting I can specify where generated image styles are marked as permanent?