Sometimes after capturing a new system and not sacrificing a point of interest to the dark gods, on the following turn the symbol is still displayed on the main map.
In the following screenshot it is obviously the turn after capturing the system, since the point of interest in the system already has gained an evolution point, yet the "claw" symbol on the main map is still there.
Unfortunately I cannot quite describe the exact circumstances, under which this happens, as it only happens infrequently. However I have now seen this several times. It has happened in systems containing either one or several points of interest.
Remaining symbols have always disappeared on the following turn (second turn after capturing the system)