It could be a bug, but it is not unlikely that the weird behaviour you
experience is due to the way the controls are intertwined -- the
original Fender tonestack (and its copies, like Marshall's) is not a
common 3-way equaliser design with independent filters.
If you haven't done so already, you can read about it here:
https://ccrma.stanford.edu/~dtyeh/tonestack/ and if that does not
provide a satisfactory answer you're probably best off directing your
queries at David Yeh himself.
(It is possible I messed up David's code while refactoring, but if I
remember clearly I tested my changes for before-after equivalence of
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