Pardon any salt, this jut cost me a game. Not sure how to replicate, so I'll provide the context.
Ending Team: Master Yi (Ocean buff), Annie (Mage hat), Brand (Ocean Buff), Kindred (Mage hat), Vieger, Malz (mage hat, Guinsoo, BotRK), Syndra (Thief's Glove), Thresh (Chain Vest). All 2-star. Level 8. About 12 health.
Before Dragon round, replaced Nautalis with Master Yi (1-star). Had 6-mage bonus.
Had full 6-mage bonus during Dragon round.
The mage bonus dropped to 5 (from 6) before the next round started.
Pre-round steps: Put BotRK on Malz (Got it from Dragon) [I'm suspect this was involved)
Rolled about 20 gold (Been hyper rolling since dropping below 30 health), bought Master Yi and a few others.
Upgraded Master Yi using Neeko's help.
Saw the set bonus was lost.
Benched and replaced Kindred, Annie, and malz before match start with no fix.
Confirmed was not a visual bug when multiple spells failed to double-cast during the round.
Managed to Test the double spatula next game. Didn't have full set bonuses (or same team), but was able to Stack Mage cap and BotRK on Malz again. No effect on the set counter (Still got +1 Shadow, Summoner, Mage, and blade-master).