The bug

Since entities' scores are serialized by their UUID, entities with the same UUID in each dimension share their score and cannot have their own score.

How to reproduce

1. Add a scoreboard objective mc-184266.

   /scoreboard objectives add mc-184266 dummy

2. Mark the chunk [0, 0] in the overworld to be force loaded.

   /execute in minecraft:overworld run forceload add 0 0

3. Mark the chunk [0, 0] in the nether to be force loaded.

   /execute in minecraft:the_nether run forceload add 0 0

4. Add an item frame with UUID 0-0-0-0-2cfca in the overworld.

   /execute in minecraft:overworld run summon minecraft:item_frame 0 0 0 {UUID: [I; 0, 0, 0, 184266], Fixed: true}

5. Add an item frame with UUID 0-0-0-0-2cfca in the nether.

   /execute in minecraft:the_nether run summon minecraft:item_frame 0 0 0 {UUID: [I; 0, 0, 0, 184266], Fixed: true}

6. Set the overworld entity's score to 1.

   /execute in minecraft:overworld run scoreboard players set @e[x=0, y=0, z=0, dx=0] mc-184266 1

7. Remove the nether entity.

   /execute in minecraft:the_nether run kill @e[x=0, y=0, z=0, dx=0]

8. Get the overworld entity's score.

   /execute in minecraft:overworld run scoreboard players get @e[x=0, y=0, z=0, dx=0, limit=1] mc-184266

   → Can't get value of mc-184266 for 00000000-0000-0000-0000-00000002cfca; none is set