Sulfur cubes can despawn after absorbing a block then having it removed

Dosnt neccesary seams like a bug too me,
All Mobs can despawn if they arent Nametagged, Sulfur Cubes should be no different.
If anything that one from a bucket that has no block in it dosnt despawn is the bug, rather than they can despawn when they dont have one.

On Java not all mobs despawn. You are thinking of bedrock, my report is under Java edition. The bug I was reporting happened as described.

I mentioned the bucketing because giving a block to the sulfur cube should have the same effect, making the cube persistent. I was trying to be specific on exactly what was happening.

If you wanna be technical about it the following mobs will be marked persistent:

• are a passenger to another mob, a boat or a minecart.^[7]

• spawned as part of a generated structure.

• have had something added to their inventory, including having something dispensed upon them (such as a saddle) or something they have picked up, but never for anything they spawn with. This includes dolphins playing with items^[verify].

• have been named with a name tag. However, one created from a renamed spawn egg does despawn as normal.

• have had the NBT tag {PersistenceRequired: 1b} set on them, whether by being summoned with it, or by being set manually with /data merge or /data modify. This is also the only way to prevent wandering traders from despawning.

(Source minecraft wiki)

The behavior of the sulfur cube might look close to „picking up inventory“ but infact it is similar to the endermans despawning behavior. The Enderman is prevent from despawning if they hold a block. Similarly the Sulfurcube reasonably should only be prevented from despawning when holding a block and after removing the block is fair game to be despawned again.