To reproduce:

1. Type /execute as @e[type=minecart] at @s run summon item ~ ~ ~ {Item:{id:"minecraft:stone",Count:1b}} into a repeating command block in the overworld and activate it.
2. Go into the end and place a minecart at the top of the exit portal.
3. See that no items are spawning.
4. Go back to the overworld, and see that the items have spawned at 0, 0 in the overworld instead.

What I expected: The items are spawned by the minecart in the end.
What I got: The items were spawned at the minecart's position in the end.

Possible ways to fix this issue:

1. Have the at subcommand redirect dimension as well as location. (Most simple fix.)
2. Add in a new "dimension" subcommand, with a value for dimension, which would redirect dimension instead, and would give mapmakers more choice in where they want their commands to run.

Ex:

execute dimension <nether, end, overworld> -> execute