ADM3485 RO output issue

I'm using ADM3485 and found one problem, could you please help to look into it?

The schematic is shown as below:

The waveforms from the oscilloscope are shown as below (DI, DE/RE, RO, B and A):

The problem is 1v voltage drop for RO signal marked by red ellipse. That is, when DE=RE=1, ADM3485 transmitt data, RO will have 1V drop.

When I removed the pull up resistor at RO, the waveforms are shown as below, the voltage drop is much larger:

When I change the pull up resistor at RO to 1k, there is no voltage drop at RO:

My question is: when DE=RE=1, what’ the output state of RO pin? How to explain the phenomenon observed?

Thanks,

Susan