Is there any way to sort of lift it up and give it a spray on the roots with plain water maybe after it floods (would that dilute the dose it receives)? Hell I dont know honestly. The plant could simply be storing more internally than the others so it's burning it some/more. Is the leaf mass about the same? Root mass? Assume so. It could be natural and it seems like all else is equal.

And again just so it's asked, you did calibrate the pen first? They definitely need it right out of the box. My meter did anyway, it was super far off when brand new.