Metaballs are a bit special, from what I understand, if you want to select only one metaball in your metaball system, use the thin line around the metaball object, don't select the metaball itself, otherwise it will select all the metaballs of the same system:
Also, as precised by Vklidu, don't delete the first metaball otherwise its children will be deleted as well. And if you change the name of a metaball it will exit from its system.
Note: The thin line will disappear and be rendered temporarily unselectable if the Collection (aka Layer in earlier versions of Blender) that contains the metaball is not currently visible, even though the metaball itself may remain visible in such circumstances.