I found a small bug regarding the named sections. Here are the steps to reproduce it:
create a box and a joined curve
set the box and one part of the curve (cmd+shift) as a named selection
explode the curve - this leads the named selection to have only 1 instead of 2 items
now Undo everything (cmd+z) and your curve is joined back together, but the named selection keeps having only 1 item, although it should have two again…
When you recorded your selection your curve was not exploded. You subobject selected a part of the curve. This means your record is 1 box and 1 sub object.
When you explode the sub object is now a full curve and not a sub object, thus it does not come back.