This occurs on the web app along with sidebar, since both are zoom-able. Default 100% zoom has no issue. When zooming out to 90%, 80%, etc. instead, the separator lines between bookmark entries can disappear from view. Likely a render scaling issue due to 1-pixel-wide lines being "shrunk" to slightly less than 1 pixel due to the page zoom.

See in the attached example, between the "Paul's Hardware" and "Bitwit" entries". This is only at 90% page zoom, but the issue can get worse at lower zoom levels.

Related to UI issues in general, zooming is required due to lack of compact mode and elements being too large in 100% zoom.