In Changelog 1.18.0 ( bedrock ) the "Known Issues" section is written:
New world generation in this release is currently incompatible with the 'Creation of Custom Biomes' experimental toggle. Expect worlds with custom biome generation to be potentially unstable and for custom biomes to only exist in currently saved out areas of the world.
I just tried, but it still doesn't work 😞
This no longer happens in 1.17.1.
Also happens in 1.17.1-pre1.
Scoreboard entries of actual entities are usually never persistent past the entity's death. The only way for the "user“ to make persistent entries is by using an explicit score holder name in a command.
The fact that a score with the name being the uuid of the – now dead – holder remains clearly breaks this rule. The "user“ never created a score under that name and it can be almost impossible to reference these leftover scores.
I have not tried this in recent versions, we should investigate whether this bug re-appeared. If so, this would need to be re-opened.