Hi Shaun, looking at the pictures, that's definitely a pot, just a different shape. It looks like it might be an Alps, (manufacturer), pot. Not sure if that will be the original pot or if someone has put in a replacement. I'm afraid I haven't seen enough different versions of that pedal to make a judgement. I do have a Goodrich pedal, the original pot in it was the silver round type but they may have used different types in different generations of their pedals. Perhaps someone here who has more experience of them could verify that.
As to your actual problem, has the pedal always done this or has this just started happening recently?
The only things I can think of without seeing it in the flesh is as the pot will most likely be logarithmic, it won't respond the same over the full travel of the pot, i.e. the resistance varies over the travel in a logarithmic fashion, so the travel works best over a certain area of the resistance taper. Sorry if I'm not explaining this very well, I don't know how much you know about electronics, so I'm trying to keep it in layman's terms.
The string may have slipped or moved so that the pot is not working over the correct area of the resistance taper.
If it's always been like this, it's possible if it is 2nd hand that a previous owner has replaced the pot and either not set it up properly or used a pot that is not suitable for purpose.
Might be best trying to find someone local who has experience of electronic repair, ideally who also knows how these pedals work. The string method kind of complicates things as it is a skill in itself to get the string put in correctly.
Here's a link and a YouTube vid that may help.
https://www.gregcutshaw.com/Goodrich%20120%20Pot/Goodrich%20120%20Pot.html