FOH fx returns being affected by monitor surface

We are running a S7000 FOH and C1500 at the monitor position with a DM64 mixrack.
All channels are duplicated and everything functions correctly with exception of one thing,
when vocal reverb is used on the monitor surface it changes the FOH setting.
There are separate send and return channels for each surface and the fx plugins seem to be routed to the respective channels.
Whenever you add vocals to the send channel on the monitor surface you get nothing in the monitor surface return, instead it is being sent to the FOH return.

any ideas?

There are no separate FX blocks per surface. All processing including FX is done in the mix rack. The 16 FX blocks you can access via FOH surface are the same 16 FX blocks you can access from monitor surface or any other surface. You have to use other FX blocks for FOH than for monitoring, like FX 1 to 10 for FOH and the rest, 11 to 16 for monitor.

If you did it that way already I’m sorry I misunderstood your problem.