I am entirely F2P and noticed the same issue. I'm at level 62 in one year, and have built up a few level 60 champs with only one fully booked. I kept going up against level 80+ every single battle.
Didn't even bother to go in, then late December I tried something differently, if I was matched with another team that was above 80, I simply forfeited. Generally by the 3rd quick loss the last 2 matches generally got easier with teams closer to 60.
Then, in January I've noticed that many of my matches are around my Level, with an occasional mismatch thrown in. Maybe Raid changed the algorithm or not, but I'm now able to win matches.