A narrower follow-up, since the general answer is now clear:

What did you change at the same time, and can you separate the two now?

SteveThurs said:

One practical note: write down what you did and when, before you need it.

Pushing back on SteveThurs here. The objection nobody has made: this only works if the measurement is doing what we assume it is doing, and that assumption has not been tested here.

SteveThurs said:

One practical note: write down what you did and when, before you need it.

Correct, and worth adding that it is reversible. Nothing being described here is a one-way door, which changes how cautious it is reasonable to be.

Happy to go further on any of that.

SteveThurs said:

One practical note: write down what you did and when, before you need it.

This answered a question I did not know how to ask. I will report back once I have actually tried it.

Clinical perspective, offered as context rather than as advice. The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either. Those three things can all be true at once, and most arguments here are two people holding different parts of that.

I would rather be corrected than agreed with, if it comes to it.