I did a little more digging as well with similar results. The help files say that "all trig functions use degrees", which worries me that they were never intended to use parameter units to begin with. I did the below with the following results:
trig_on_degrees=tan(angle_degrees) predefined parameter with units. BAD
trig_on_radians=tan(angle_radians) predefined parameter with units. BAD
trig_on_none=tan(angle_none) predefined parameter no units. GOOD
trig_on_feature=tan(d13) feature with undefined units of degrees. GOOD
deg_rad=15[rad] new parameter, incorrect units defined on the fly.
trig_on_degtorad=tan(deg_rad) GOOD
rad_deg=15[deg] new parameter, incorrect units defined on the fly.
trig_onradtodeg=tan(rad_deg) BAD
Finally, I tried switching to several different sets of system units and nothing changed. I was hoping that perhaps the problem was that the parameter units are different than the system units so they are automatically converted. So switching the system of units to something compatible with the units in the parameters would keep it from being converted. Either there are no system of units with degrees or else something else is going on.
So, until they fix their software, it looks like for trig functions you either have to specify no units or incorrectly designate them as radians (15rad instead of 15deg). That's really messed up.
I'm on Creo 2.0 M070 as well.