My advice is contrary to what Piotr does. Delete a symbol only if you want to delete it permanently. If you want to for example change to an alternate symbol but basically keep the part the same or at least functionally similar, use “Change Symbol” context menu item. Learn the workflows which allow you to keep the “Re-link footprints” option unchecked as much as possible.
For some details about this PCB update process, see Update PCB from Schematic's match methods.
You will end up to the situation you see now for example if you delete that bunch of symbols from the schematic and then add them as new ones again, or move them between hierarchical sheets by cutting and pasting. This may also depend on the annotation settings.
This may sound complicated at first, and to a degree it is. KiCad works this way. Fortunately it’s not difficult to learn how it behaves, and you will soon know how to fix things if some situation arises.