I have this issue with foobar2000 and George Yohng's VST Wrapper.
I notice that when the footswitch is toggled off the output volume of the OwlSim is lower than expected. When I bypass the VST in the wrapper, the output volume is as expected.
I tested NRR-1 VST Preamp, set the preamp off, engage/bypass the NNR-1 VST and the volume do not change between the 2 states.
With Reaper this is a non issue, I noticed no volume drop.