Hi Anna
could you be more specific about "after a while" ?
What do you do in the meanwhile ?
taxi / flying away from the objects ?
was the objects out of field of vision, then missing when you looked back ?
that being asked ...
to address your question "why" without further :
1.
in FG menu
View / Adjust LOD details
in upper part of modal window,
one can choose granularity of visible objects based on distance.
Ascertain to have theses slider to have objects in range.
wiki
https://wiki.flightgear.org/Level_Of_Detail_(LOD)_Ranges2.
objects may have in their .xml definition file
a range specification
https://wiki.flightgear.org/Howto:Animate_models#Range3.
FG use a mechanism to unload from memory unused part of the scenery.
This mechanism have been updated.
Improvements are expected
https://wiki.flightgear.org/PagedLOD