Hey guys and girls.
I just upgraded to v3.0.5, and one of the things I often did in v2.x.x is bulk update for adding categories to products. For example, I have 40 products and I need to add a second category to all of them at the same time, instead of going one by one.
It used to work fine, but now when I choose all my products, then click “Edit selected” → “Apply values to all the selected products” - > Choose category → “Apply”, it changes all products to that category BUT deletes all other categories that was associated with it before.
I tried it on different products, and it happens every time.
Anybody else having this issue? And any idea how I may be able to fix it?
Please perform a test installation with demo data. Then attempt to make bulk modifications to products. Make sure you change the “SECONDARY” categories field, and not the primary category field 
I hope you have already formed a habit of performing a backup prior to any store updates you perform…regardless of how small…?