Has anyone noticed VO2 Max going down despite increasing cardiovascular training? Loose watch band cause?

I have noticed my VO2 Max going down for months despite increasing cardiovascular intensity. It seems there is no convincing explanation.

However, my VO2 Max just climbed quickly back up (the week I had COVID, which I think is a coincidence) when I switched my Apple braided solo loop watch band (which had been loosening over time) to a normal clasp watch band. Time will tell but it could be the cause; maybe the loose fitting band creates poor signal which affects the algorithm to calculate a lower VO2 Max. Thoughts?