Exactly. The saw blade is in the same position,
the stock is square.
The magnification doesn't happen if the stock is square.
The blade is square, hence the perpendicular cut.

wish I knew why this happens.

Swingman wrote:
"Leon" wrote

You need to use a blade that cuts perpendicular to the work to have a
taper begin or end squarely. Try your bandsaw.

Still doesn't explain why, in the OP's original query, one cut on the TS
will be square/perpendicular, and the next not?

( I usually sand my way out of this same situation when using a taper jig,
but still experience the phenomenon occasionally myself.)

Obviously, the juxtaposition between the blade, the stock, and the jig
(stock always varying, however minutely, in dimension/flatness throughout
its length) is at the root of the problem, but I'll be damned if I've
figured out how to overcome it myself when using a tapering jig on the TS.