It also isn't necessary when the group is expanded
The visual metaphor used for bundles was about notifications being stacked on top of each other. The line is a subtle indicator that these are not a single notification but a stack of many.
When expanded, it seems that the additional line is still visible, and as noted by @Esanders, it should not be the case.
That line generates a separation which is intended to be consistent with how the notification opens (notice in your example of F4339766 the bottom white element that helps separate what's inside the notification from what is outside, especially when the next one is a read one).
We can explore other options, but I'd keep it in the subtle side since the "expand" label and icon already communicate clearly the functionality.
It is a light grey line on a lighter grey background, so it may not be easy to notice depending on the contrast settings of your screen. I tried to zoom it below to clarify:
If my DOM inspection is correct, it is the "mw-echo-ui-crossWikiNotificationItemWidget-separator" element which should be visible when the bundle is collapsed but hidden when expanded.