Do you mean Arena Assault? If so then yes, I've noticed that as well.
I only do the first run at full power to get the seven consecutive wins, then every other day I just surrender at the beginning of the fourth battle. So in order to make that easier I used to start with my flier team which gave me unmerged enemies from levels 35-40 4/5 and then I just finished the rest easily. I mean, I just wanted to get the feathers and I'll surrender at the 4th match so there is no need to look for stronger foes.
However, I keep getting only Lv.40 5* units and also as you said at least half of them are merged. Which is weird because I pick Beginner to finish as fast as possible.