It is probably due to the fact that you have not connected up the VCC and GND to the two modules. The initial communications transitions are back feeding the VCC and GND through the input protection diodes of the MAX485 until the C1 bypass capacitor gets charged up to a stable value.
Make sure not to run your RS485 transceivers without power applied.
A real RS485 bus would be setup for bidirectional transmission and not dedicated to one way flow. If you want to use unidirectional flow you should be more apt to study termination techniques for RS422.
Symmetrical parallel termination at each end like you show is what you want for a bus that needs to transmit in both directions.
For unidirectional ...
Pull-ups on the A/B lines mostly do not depend on the bus length, they are there to provide a proper logic level for your RS-485 transceiver when there is no active driver on the bus, in case the bus is open, and in the short circuit event.
Most driver in the past had a differential threshold voltage level from \$-200mV\$ to \$200mV\$, so that when you have ...