RS-485 works when USB powered, not when main powered
I used the circuit from RS485 (MAX489) Biasing and it all works now. Communication with my modbus power meter works flawlessly.
Basically, I added a 470Ohm pullup on A and a 470Ohm pulldown on B.

I reminded I bought the max3485 long ago on a big Chinese online shop. So my so called max3485 may not be a real max3485.