Weird item changes by level up your champion

I would like to discuss weird mechanics (probably bug) about leveling up your champions. Let me describe situation I was in:

1. I had a 1 - star champion with 3 fully combined items (static shyv, guinsoo, gunblade)
2. I needed to level him up so I took that champion during carousel round, so that taken champion had recursive bow.
3. I bought another that champion from reroll shop, so it leveled up.
4. Weird mechanics start here: Somehow it leveled up but dropped early combined guinsoo and changed it to recursive bow which I had early on one of the champions I used to level up.

P. S. All in all, that situation resulted in losing fully equipped champion (I needed one more item to make that guinsoo again which ofc I hadn't got), which lead me to lose fast (it was mid game, so I couldnt change anything fast. Also: this was my main damage character).

Have you ever been in that situation?