HI there, I will try to be as clear as possible 
The following happened to me:
One of the Categories I created was called “Keys” with two tabs “Dungeon” and “Arena”, after deleteing another Catergory that was empty, my entire Key Category with all Tabs and Items disapeared. No biggie as you get flooded with keys anyway 
This can be prevented by paying attention, but it’s still a bug
You can reproduce this by:
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I created Categories and Tabs in my Stash, like you can see in Pic1
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In Pic 2 you can see that I created additioanl Categories. Pay attention to Category 6 for this exercise
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Pic 3 shows me right clicking on Category 6 with the intention to delete it. Here is where the confirmation window shows “Exalted” and that would be indeed the category and all it’s items would be deleted (just what happened to my Keys).
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Pic 4 Shows that if create a new Tab in another category (as I can not creat tabs in the new categories) and try to move it, to Category 6 in this case, it ends up staying where it is.
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Pic 5: if I switch to Category 6, I end up seeing the Exalted category with all it’s content.
So it appears that the categories are wrongly linked in the database and assiciated. I am literally unable to do anything with new catagories and there is danger people jnot paying attention to what is deleted for real when trying to delete a category.
I hope this is clear enough.
Cheers,
Odlanir
all pics in one plce: https://imgur.com/a/1EgcSrh