So it seems like the problem is still there however having checked through various devices and browsers it looks like the issue only lies when people use Safari Web browser as far as I can tell.
I have tried;
Windows-Edge, all fine:
Mac-Firefox, all fine,
IOS14-Safari, plus and subtract buttons still don't work even when i clear history and cache. It works the first time you load the page but refresh the page or go back to it later and the buttons don't work. Same issue with Mac-Safari.