IIRC, the lower ball on an s5 shifter is small and uses a nylon bushing in the extension arm cup.
http://www.mazdatrix.com/g4.htm
S4 shifters have a
larger lower ball and uses
no bushing. Is there play where the lower ball and cup interact?
It could be that when moving over to 1-2, there isn't much force on the spring as there is play between the lower ball and the cup.
When you move over to 5-6, the tension is on the coil spring, and you still get rebound..