Phone: Redmi 4A
This glitch happened using the classic UI setting
When buying an unstackable item such as tools, armor and books from a villager, buying a second item and stacking it on top of the item that was last bought will cause the last bought item to disappear
Steps to replicate this bug:
1. open the trade interface with a villager and switch to a trade for an unstackable item (enchanted books, tools, armor etc)
2. fill in the emeralds and other required items on the left side of the trade
3. tap on the item icon on the right side of the trade to buy it
5. tap on an inventory slot to put the bought item there
6. tap on the item icon on the right side of the trade again to buy another
7. tap on the item in the inventory that was just bought. The last bought item will vanish

I'm unsure if the bug is also present on the "pocket" UI since I was playing using the "classic" UI.