Wierd bug with the group

I have a very wierd bug with my cubase software :
i have 2 guitars each of them panned left or right accordingly
they are both sent to a guitar group.
in that group i use the cubase eq and stl tonality plugin in the inserts.
the guitars are playing the exact same line except for a little segment where they play harmony together.
for some reason when the guitars channel are sent to the group which goes to stereo out
i can hear only one of the 2 lines at that specific small segment where they dont play the same.
if i send them individualy to stereo out or even one of them to stereo and not the group , i can hear both lines.
the stl plugin is not the problem here and the group was opened as a stereo group and i can hear harmonies in other parts of the song.
i tried reopening the project and making sure my cubase 10 is up to date with all of my hardware also updated including soundcard, windows and BIOS
please advice.

Hi and welcome to the forum,

Are the source tracks mono? Is the Group stereo?

Could you attach a screenshot, where we could see the routing, please?