HP-50g derivatives

[JurgenRo]

I've encountered he following "glitch" with the HP-50g:

Storing S^2 in variable T, the command DS(sin(T)) will expand to DS(sin(T)) = DS(sin(S^2) = 2Scos(S^2), which is just fine.

Storing S^2 in in variable T as above, according to Nick K's "Calculus Marathon on the HP-49G" the command DT(sin(T)) should expand to DT(sin(T)) = cos(T=S^2) = cos(s^2). But it doesn't! Instead, the result is DT(sin(T)) = 0, which suggests that the expansion rule in this case reads: DT(sin(T)) = DT(sin(S^2)) = 0. That's bad ...

I'm wondering if there is a difference known in the defininition of D (derivative) in the 49g Firmware and the 50g Firmware (Rom version 2.15) or if I'm just missing something here?