I havenāt experienced it but i have been in an associations with 2 friends who ended up in the same league. The one who does not stand a chance gives way to the one who would have won anyway so that he could use his main team for the other competitions for that particular day. Thats what i got when i translated their conversation in the associations chat. It didn't seem like it was cheating when i looked at it from my association mateās perspective as he felt he would have lost anyway. I too felt he would have lost. Similar quality teams but one was well trained while the other had high greys. Also high greys team hardly had full 40 training bonus, always hovering around 18 to 28 while the other team was always 40.
With regards to reporting, never tried it as never experienced it so sorry if it wasnāt the answer youāre looking for.