The subject summarizes the bug. I apologize if this is/was already known, but it hadn't seemed to have been.
Interestingly, if categories are duplicated and reach over the limit of fifty, all items previously in a category other than the "none" default are put back into the "none" default.
Edit 6/15/2020: The problem is still occurring.
If this is still happening to you, could you please let me know the following information:
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Okay, I figured out why it's happening to your specific gallery (but it's still a glitch on our end, sort of)
Could you try going to the "Edit Categories" page of your gallery and deleting the extra space in between the words in each category name, save changes, and see if that resolves the issue?
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