LR Balance problem

I've got an issue with my dual-sided Vector 3 pedals. Over the last few rides, the L-R balance has started to shift wildly from 47-53 about a month ago to 39-61 today. I do have a knee issue but its been stable around that 47-53 level for a couple of years so I'm sure it's not related to that.

Does anyone have any ideas what the problem might be?