Adding new layer to existing group

I’ve got a group layer with two layers and want to add another layer. To be specific: I’ve drawn a line on a map, grouped it with the map, moved the group on the canvas and would like to draw another line, again linked to the group rather than making another group with the existing group and the new line. All this is done in edit-mode. When I try to draw the new line into the existing group it disappears although it’s on the top of the stack. How can I add the new line layer to the existing group such that it doesn’t disappear?

Did you check Blend Mode of the new layer?