How VFD affects an isolated IOT system
There could be a few reasons here. The disturbance may even be radiated rather than conducted on A and B from your circuit's point of view.

As a simple place to start; your drawing doesn't seem to show a shared ground connection from the isolated side of the RS485 to the remote terminal. Without that you are in the realms of 'might work' and you may be exceeding the common-mode performance of your transceiver.
An alternative (worse) approach to a shared isolated ground reference is to have 'leak' resistors from A and B to your local signal ground, fail-safe biasing resistors can create the same result too.