Thanks for your question. It looks like you are trying to use the Gutenberg “Gallery” block; is that right?
The “Create Gallery” popup window has a dropdown just below the “Filter Media” heading in the upper left corner of the window. I assume that’s the control you are referring to; is that right? This dropdown control is a WordPress feature, not an MLA feature.
On my system, with MLA enabled, this control is initially blank but if you click on it you get a list that includes “Images”, “Uploaded to this post” and “Unattached”. On my system, all three of these options work as expected. In particular, “Uploaded to this post” shows the images that are “attached” to the post, i.e., their “Parent” is the Post in question. You can verify this by looking at the Media/Library “Uploaded to” column (in list mode) or the Media/Assistant “Attached to” column. WordPress does not always “attach” an item to the post when it is used as a Featured Image or when it is inserted in the body of a post.
You wrote “I have the settings “Featured in” and “Inserted in” ENABLED and as I said, is work perfectly with the normal editor.” These settings only apply to the MLA “where-used” analysis; they play no role at all in the population of the Create Gallery items.
If you have verified that images attached to a post are not showing up in the Create Gallery window you can try disabling MLA’s enhancements to the window. To change the settings, go to the Settings/Media Library Assistant General tab, scroll down, uncheck the “Enable Media Manager Enhancements” checkbox, then scroll down and click the “Save Changes” button. If you do that, does the problem go away?
If you are not using the “Create Gallery” block, let me know the specific block you are having trouble with so I can reproduce your issue on my system.
I will leave this topic unresolved for now. Any additional details you can provide will be helpful. Note that I will be traveling next week and unable to respond until I return. Thanks for your patience and your interest in the plugin.