It appears that at a certain point this mod set all of my ore fields to only produce exactly 0.5/s of ore. It's infinite at '100%' yield, but those fields were producing way more than .5/s per miner.

Is there any reason why it would do this on purpose, or is this a bug?

This mod doesn't do anything to cause that, I think that's just how infinite ores work. They produce ore at the same rate no matter how much coverage your miner has. I guess the logic behind that is that infinite ores are much more powerful in other ways, so the fact that you need more miners to get the same rate balances out the fact that they never run out. Again, this mod doesn't specify any of this, it's just how infinite ores are treated by the base game.

pretty srue that the mod only ever changes ore yields. I noted that yield dropped off super quick after upgrading yields with miners via research. that's probably what it is.