I have same issue with missing picture on thumbnails (my gallery has over than 100 images) and I saw here Robosoft asked to download the fix script in this folder below,
wp-content\plugins\robo-gallery\cmb2\fields\gallery\js
Unfortunately I couldn’t find gallery folder in my fields folder? nor new update plug-ins? I have rbsgallery folder in the fields folder.
Can you guys point me onto right direction please to fix this issue? Thank you.