Well, I have some thoughts. But there is no real answer out there yet for this type of question.

You are taking 6mg a day.

I search B6 alot and have never found ranges for those people who consume B6 before the testing. Somewhere there must be ranges for people using supplements, I just haven't found them yet.

The ranges are statistical variances based on "normal" people. And who defines "normal"...no one.

I guess one way to find out more would be to have a homocysteine done. Since B6, B12 and folate work to lower this substance and one or more may be failing to be activated, then homocysteine will be elevated.

The B6 test measures all B6, not just activated B6. When the serum is high, we don't know if the tissues are correspondingly high as well. If B6 is not being activated to P5P then it will read high I would think as it builds up.
The enzyme that converts B6 to P5P is pyridoxal kinase, and this can fail.

Are you seeing any changes in the urine..as B vitamins often cause a yellowish green tint when they are excreted.

Your lower B12 may be due to the lower animal meat you are eating too.

There is just very little research on changes like this. It could be that people vary quite a bit, and testing never is done commonly so it is invisible and we don't know it is happening.
It could be we change as we age. It could be your B6 is not being activated. It could even be a fluke, and part of a false reading by the lab. Lots of variables.

There is a situation where blood work is not handled properly and the red cells burst...releasing substances. The most commonly understood is the factitious readings for potassium which are affected by this mishandling. It is possible if your sample is mishandled and the red cells dumped their pyridoxal content into the serum, the reading then would be falsely elevated like it can be for potassium. I haven't seen studies on this issue except for potassium, because I don't think they have been done.