Steps To Reproduce:
Summon an item using
/summon item ~ ~ ~ {PickupDelay:-32768,Thrower:{L:0,M:0}}
Run the command
/execute store result entity @e[type=item,limit=1,sort=nearest] Thrower.L long 1 run data get entity @s UUIDMost
Run the command
/data get entity @e[type=item,limit=1,sort=nearest] Thrower.L
-> See that the 'L' tag is 2147483647, not your UUID, which is about twice as long.
This is probably caused by 2147483648 being the max value possible for /execute store, because that's how much scoreboards can store, but the game doesn't realize that numbers higher than this are possible for tags like "Thrower" and "UUIDLeast".
Command result (and as such the highest/lowest possible result value) is an integer, and that’s not changing.