All screenshot thumbnails are not visible as we open Jira issue for the first time. It is available on a page refresh though.
there should be an error logged in atlassian-jira.log in case the thumbnail cannot be created - please check if it is the case.
As a random example: in https://jira.atlassian.com/browse/JRASERVER-45340 an error message is available - I would be on the lookout if such is available in your logs.
The good news: the bug linked above itself does not affect you - in case a thumbnail cannot be created Jira tries again to do so. From what I understood after a refresh it works - so basically the "mechanism" is ok.
Please try to have a look at the logs in the first step now...
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