If a single standing Sculk Block containing a sculk charge is destroyed, the sculk charge will vanish. The sculk charge will not remain forever, regardless wether it is currently spreading or not. However, if a Sculk Block is around, it will accept the charge. Spawning Zombie from a spawn egg into Lava located 1 block underneath close to the catalyst will not cause any spread. There are certain conditions required for the Sculk Catalyst to spread towards other blocks. Sculk blocks allow the charge created by sculk catalysts to travel through them, allowing the charge to convert blocks further away.
Instructions: more information on how exactly sculk charge spreading works
