When using @s with the target selector distance, the dimension used for the distance selector seems to be overwritten by the @s selector.

For example,

execute positioned as @s in minecraft:overworld if entity @s[distance=0] run say "I'm in the Overworld!"

and

execute positioned as @s in minecraft:the_end if entity @s[distance=0] run say "I'm in the End!"

both run if you are in either the End or Overworld.

Curiously, dimensional coordinate systems are still in effect;

execute positioned as @s in minecraft:the_nether if entity @s[distance=0] run say "I'm in the Nether!"

only works in the Nether, probably because of the coordinate conversion.

I was doing this to obtain the dimension of mobs (by executing as the mobs using a data pack function) since the Dimension nbt for them has been removed since 1.16.

A temporary workaround is using @e instead of @s for the second selector, but that comes with the side effect of possibly detecting other entities that just happen to be in the exact same position in a different dimension, and probably being a more resource intensive command.