Is it that radar can't ping / receive at a high enough frequency to distinguish the difference between "this fraction of a ms the blade was at that location so we don't care about the radar hitting something, but the next fraction of a ms the blade had moved and we still got a ping from just behind it so there's something there"? Or some other problem with the idea?
Reflections from targets also need to pass the turbine(s), reducing the return even more.
Beamwidths are significant, depending on the antenna configuration. You can have beamwidths of 0.5 degrees and more. That helps detection, but reduces target discrimination. It helps get the energy past the turbines, but you tend to have "wound down" the power output to allow for that. So it doesn't really help.[1]
You know where the turbines are ... knowing the exact positions of the blades won't help at the frequencies being used.
In short, it's complicated. Radar pulses are like HUGE lumps of energy flying through space, and delicate things like turbine blades chop them up and defect them around, preventing them from getting to, and then from, the targets.
[0] Marine radars, X-Band and S-Band. There are other ranges and bands.
[1] Yes, this is all extremely simplified and inaccurate, but I'm trying to give a sense of what's going on.
There are trade-offs between how often you can ping versus how far you want to see versus how much energy you can send out versus how big the "lumps" are, etc. The sums/calculations are pretty simple, but not obvious until you've seen them, and then they are.
But if you want to see something at 32NM ~ 60km, you have to send out a lot of energy, so the "lumps" need to be pretty big, especially as the returns drop off as a fourth power with distance.
E.g. are the US military saying "yep those are the issues we face too" or "haha our tech has more than enough energy to get around all these problems"?
I wouldn't care to speculate further, but you might want to read about phased array antennas, and FMCW radar. There are some very deep rabbit holes here, and I certainly would not be your best guide.
Plastic drones with plastic propellers are still visible on radar because the tiny propellers spin super fast, so they light up like a Christmas tree on Doppler radar because the approaching vs receding velocities of the blades are so different.