it’s this class selector .workspace-leaf.mod-active, which is only applicable when that particular pane has the focus. in your case, this is what makes the border as such. i guess removing it is sufficient.
.workspace-tabs will make it apply to both leaf (left and right) but not the note’s pane/leaf.
but i guess u need an alternative for users to know which tab/pane is active. just good ui/ux practice.