This is a bug in 2020.3 also, and for all 1.15.2, 1.14.4, and 1.12.2... tested it on all versions, is what I'm getting at.
What happens is when you have an advancement for obtaining an item, (and the default setting is having 1 to 1 items).. You will only get the advancement in game if you ONLY have 1 item in the stack, if you have 2 or more in the stack, then you do not get the advancement..
This doesn't happen with vanilla items, you still get the advancement when you have more than 1 item in the stack.
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