Steps To Reproduce:

1. Summon an item using

   /summon item ~ ~ ~ {PickupDelay:-32768,Thrower:{L:0,M:0}}

2. Run the command

   /execute store result entity @e[type=item,limit=1,sort=nearest] Thrower.L long 1 run data get entity @s UUIDMost

3. 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".