Issue with mono send bleeding to mains right channel - Cubase Pro 10.5.2

Hi, having an issue, hoping someone may have found a solution. While tracking, I have all my instruments grouped (drum, bass, guitars, keys) for the vocalist to control his headphone mix while tracking.

All groups are stereo, however, I’ve put a mono prefader send on the bass group as the monitoring doesn’t need to be stereo. Trouble is that when I enable the send on the bass group, it doubles the level of the bass in the right channel of the main L/R outputs (both phones and speakers). If I pan the group hard left it’s balanced and mono, though the right channel is unprocessed. None of the meters within Cubase reflect this right channel level change either. If I turn the send down, it controls the level of “extra” bass in the right main output as well.

Using a stereo group seems to solve it, however, I don’t have the ability to use an additional stereo group due to how things are routed. I’ve tried changing the send output to a number of different outputs (none of which being what is also the right main out), but I always get the bleed to the right main out. I’ve investigated in the hardware’s mixer app and can’t find anything.

I know I could use one of several workarounds to “solve” it by changing the bass to a mono group or a stereo send, but I would rather address the actual cause and correct it, as there is stereo processing on the group, but I only have a mono option for the headphone mixer.

Anyone else have a similar issue with sends bleeding into main outputs?

My setup:

Cubase Pro 10.5.20

Audient evo16 with evo SP8 ADAT preamp extension

2020 Mac Mini OS 11.7 Big Sur

How have you done this?
A send channel format is defined by the target it is routed to.
If you route a send to a stereo group it is stereo.

Are the groups routed to the main mix?