Noisy RS485 signal with MAX13487E and Raspberry Pi
The block diagram of the first page of the datasheet for the MAX13487E indicates that the driver can shut itself down based on the conditions on the line. It can do this even if you have manually disabled the receiver by forcing RE LO (i.e. turning off the receiver doesn't mean forcing on the transmitter)  since you can see RE is unable to influence the driver enable.


This probably exists so that you don't have two devices fighting each other trying to drive the line. That means that if your idle line conditions are wrong, the MAX13487E will think a device is transmitting and back off by shutting down its transmitter.

I think your 10K resistors are too large relative to your 120 Ohm and are causing your idle voltage to not be >200mV which is confusing your auto-detection circuitry and making it think another device on the line is transmitting so it reacts by shutting itself down spuriously during transmission.

I don't use auto-detection transceivers so I don't know for sure. Let me know how it works out.