Context. summon an area effect cloud nearby. let this not have any owner, and for simplicity lets not have it despawn.

I expected:
/execute store result entity @e[type=area_effect_cloud,limit=1,distance=..10] OwnerUUIDMost long 1 run data get entity @p UUIDMost
To perfectly store the players UUIDMost tag value (long value) into the OwnerUUIDMost tag (long value) without problems.

Unfortunately minecraft downgraded my long to an int secretly in the middle of the command. WHICH MAKES THE 'long' specifier USELESS.

It is expected that using /execute store on LONG values simply takes the long value from one place, and stores it in the LONG value of another without conversion.

However this behaviour also happens with scoreboards. that's cause those are just bounded by integers. Either consistently upgrade those to long values as well or leave them be.

But from Long --> Long, why degrade to an int value. This way we can't ever custom dynamically change the owner of a cloud. which limits a whole lot of creativity. or requires us to use lots of extra commands for synthetic UUID creation, but still is impossible to use automatic scoring such as killcounts.

Imagine:
How do create a custom area of effect cloud of damaging, which kills a player, that actually tracks the player who casted the spell to begin with. Impossible as it currently stands.