The reasoning behind the sorceror rule seems to be:
Before casting the heightened fireball in the 9th level slot, there is no 9th level fireball, so you can't get the 9th level "fire" bonus for your "fire" reserve feat.
After casting the heightened fireball in the 9th level slot, that fireball is gone, so you can't get the 9th level "fire" bonus for your "fire" reserve feat.
I would see a parallel with the cleric. If they don't have conjuration (healing) spells prepared, then:
Before dropping a different 4th level spell to cast a "cure critical wounds", they have no conjuration (healing) spell prepare, so cannot use the benefit of the "healing" reserve feat.
After dropping a different 4th level spell to cast a "cure critical wounds", that spell is gone, so they cannot use the benefit of the "healing" reserve feat.
The parallel seems pretty strong to me.