Higher tiers may be having the issue of having fewer players in it as well, meaning the algorithm would have less options to choose from. E.g., say it picks opponents from within +/- 50 arena points, and then tries to match against similar power levels, but it runs out of viable opponents and so is forced to pick opponents that are not similar in power level. There may be other criteria, like a safeguard against someone getting picked for matchmaking too often, that may influence it as well by disqualifying many of those near you in arena points and power. In B4-S2 which is the pool I'm swimming in, I'm guessing there are a lot players to pick from.
And actually, yes, I would very much prefer random matchmaking within the same tier (or within an arena point range). Why? Because it means that matchmaking will be based entirely on your performance in the arena, not some arbitrary number. I made a couple of suggestions here a while ago, but to summarize the core points: 1) pick random opponents within same tier/arena point range, 2) whenever a team is selected in the matchmaking, give it a point. Yes, you'll get a mix of incredibly easy and incredibly hard opponents, especially at the lower tiers, but so what? You pick the easy fights, crush them, and move up until the fights are no longer quite as easy. Likewise, if you're far too strong for the tier you're in, no one will want to fight you and you'll just float up until you reach a tier where there are people who will.
There may be some adjustments required to prevent arena point inflation (e.g. a natural decay), but ideally everyone should be in the tier where they belong. Account-power based matchmaking may protect new accounts from getting crushed by far too powerful opponents, but it likewise drags down people to lower tiers because they haven't optimized their account power. Considering faction crypts is a game feature that is built around collecting as many heroes as possible, having a system that punish you for trying to make use of it is a strange design choice (and likewise, if you want to use the arena then you're punished for wanting to do crypts). Instead of trying to shield low-power players from high-power players, ensure that those high-power players are not in an arena tier where they don't belong in the first place.