Around 2016 we started adding a resistor to provide a weak return path to ground because some units would develop this condition you described over time. I think the problem stems from the fact that there’s no return path to ground when the attenuator is 0 dB, and a tiny leakage current from the attenuator can essentially blind the RF switch until a ground path exists.
If you can set the attenuator to 5 dB, or add a 1 dB external attenuator, this problem should clear up. Otherwise, you can get an RMA, send it in, and we can add the resistor.