Hi
Corona DB 2021-04-21 and previous one (didn't try with Corona 6), if secondary GI solver set to 4K Cache, GI in reflections gets splotchy during IR. There is no such problem with UHD Cache. These splotches appear only during IR, no problem with production render.
On attached pictures we see the boxes on the wall reflected in flat mirror placed on the opposite wall. Here is also the scene for you, made in Max 2021, saved as Max 2018
Max 2021.3