x^y (x power y) key on HP prime calculator when running in RPN mode seems to have flipped x, y values on the stack.
In other words, for a normal HP RPN calculator: 3 enter 4 x^y will give 4^3 = 64. But HP prime gives 3^4 = 81.
Is this a bug or a feature?
It's a feature. Here's the source of your very understandable confusion:
On "old school" RPN calculators (includes the 15C), the stack had only 4 elements, from bottom to top, X, Y, Z, and T. "T" would replicate when the stack dropped to allow for calculations using a "constant." Y^X on those calculators was labeled as such because you would first enter your base then the exponent followed by the operation, leaving the Y argument in the "Y" stack register and the "X" argument in the "X" stack register. Hence 3 Enter 4 "Y^X" would give 81.
On the new calculators, there is a massive stack (something I personally saw no use for--I never ran out of stack space doing calculations on the old stack EVER in decades of advanced calculations). It made the prior constant calculations impossible on the new calculators without programming. The elements are now simply numbered 1, 2, 3, 4, 5, etc. For entry order, they figured input order of variables should be alphabetic, hence x^y. Since there are no longer stack labels, there is no longer a contradictory model of a Y register and an X register as before, so 3 Enter 4 "X^Y" still gives 81, but there is no contradictory register entry here, just a memory of how an older machine labeled its registers. I don't think many would have mistaken things if it were still labeled "Y^X" as before, and I'm personally not fond of the change, but I can understand the motivation. It is not a bug.