When using the "at" under execute, the command will not run. This also fails if the command is run in chat and is pertaining to an entity other than the user.
Comments 3
Is it just that "at" does not set @s to the previous use of @a in the command? If so, is this a bug?
EDIT - ah, yes. Here it is in the changelog.
/execute at <entity> <chained command> executes a command using the position of <entity> (but doesn't change entity)
https://gist.github.com/Dinnerbone/943fbcd763c19be188ed6b72a12d7e65
What is the full command you are using?