Even though it is water soluble, yes it can accumulate, especially at the higher doses found in supplements. The primary way this happens is via unaware supplementation. Usually people are unaware that their product contains b6 - it's in a lot of products that are not advertised to contain it.

So even though it is water soluble it can still accumulate when taken at these high doses. Most supplements contain pyridoxine, which can acumulate and damage peripheral nerves. Indeed the form of B6 is important, and manufacturers take advantage of the fact that consumers (and medical practitioners) are unaware of the difference. Taking P-5-P may be less risky than pyridoxine hydrochloride, a cheaper option that is included in most supplements.