Sounds like you have a spring broken or missing or something is just interfering with the linkages. Have you crawled under that side with the engine off and inspected the pedals and linkage? Tried lubricating things?
The rocker pedal on my Kubota starting to get a bit finicky. I tap it and the key works but, yeah. Only 280 hrs
Suppose I ought to investigate and lubricate as well.
I would review the parts diagram too. Look at Messick's diagram under "speed control". There are 2 diagrams that might help you. At quick glance I only saw one spring attached to the neutral arm but everything needs to move freely in order to let it work smoothly.
The treddles on both my Kubota tractors get flaky this time of year. The cold weather stiffens the grease. Working the mechanism a few times with the engine off and adding more lube solves the problem.
I had that, looked underneath and found the dealership zip-tied the 3rd function hydraulic line to the reverse mechanism, and that affected both forward and reverse, unbelievable!
I had a BX Kubota that had the same problem. It was a common issue. There was an adjustment on it that helped a little bit. I unhooked the dampener on mine and it helped a lot but made the reverse stop pretty sudden. When I took a hard look at the setup and thought there was a geometry problem with the return spring. It seemed to work fine if you were going fast and let off the pedal it was a problem when going slow or letting off very gradually.