CC1101 Transceiver pair not working when relatively close to each other
Very simple: receiver overload.

RF receivers are sensitive devices, which means you've got some amplifier in there that converts a very low-amplitude signal to a signal that is many thousand times as high in amplitude.

Now, this amplification ideally happens linearly, i.e. output = k · input, no matter how high or low the input.

If you, however, have an input that drives the amplifier to saturation (for example, k = 10000, input = 0.01 V, but only 2 V supply voltage: you can't achieve output = k · input = 100 V), then nonlinearity happens, and that means (among other things) that different frequencies in the received signal mix, and the resulting mishmash of things isn't decodable.

Just like you won't be able to understand what is said if we strapped you to a concert speaker box, set the amplifier to max power, overdrive it, and deafen you with something screamed into your ear, the receiver simply can't handle signals that are "too loud".

So, yeah, reduce transmit power if you don't have the loss due to free space distance, or reduce the receive gain.