The spawn space around a spawner is not symmetrical. It is 4 blocks on two sides (towards negative x and z), 3 on the other two.
While it had been like this for a long time, I can't imagine that it is intentional. So a bug, but an old bug.
MC-4338
Spawner area not symmetrical
Resolved
Duplicate
0
This ticket has been marked as a duplicate of MC-4855 , in favor of the better description/information provided.