I bought a 16k stone a month ago and have been pulling my hair out. 8k leaves a mirror-like shine and 16k leaves deep streaks like a 1500 grit stone would leave.

Then it dawned on me - I'll bet the Shapton 16k and 30k have periodic manufacturing issues simply because a supplier mistakes those 4 zeros for 3. So, in every few batches you might be getting a bunch of grit added to a batch of 16k stones that's 1600.

Knives & Stones in Australia kindly offered to refund the stone & shipping and will arrange to get this particular stone back to Shapton to find out.

I don't think you're losing your mind at all.