Thanks both of you. @Zacke I did observe an extreme version of my issue in a SoCal flight where the OAT was around 100F and of course sunny -- I wasn't surprised by the temp skyrocketing in the afternoon sunlight. But I also do a lot of early morning and early evening flying in places where there's not really a lot of sunlight -- cloudy Southeast Alaska, for example -- that's what made me question it.
@Black-Square thanks for confirming it wasn't my imagination. To be honest, cabin temp control flummoxes me in a number of aircraft, and I might fixate on it a bit too much as a result -- probably the main reason I even noticed this, because it put me behind the curve right from the start lol