Does the dust covering the modules make it work worse? Do you clean your APs and SMs? Our main cluster is on the grain elevator and the APs covers with a coating of dust for a couple of months.
We had noticed a considerable improvement of a link with one of our clients after changing the AP which was dirty to the clean one.
I imagine the dust can produce some kind of electro-static charge that somehow can influence in the Canopy (or other) antenna system. We all know an antenna it’s an radioelectric conductor, so I think the dust can affect the signal as well as you described it.
Would be better to clean it out if the dirt is thick enough.