I'd guess your problem relates to either the dark color, heat buildup with the storm door, improper priming, or type of paint. We have a steel front door that faces SW and were told not to paint it a dark color and definitely not go with a storm door. The original steel door we painted ourselves & we had trouble with paint adhesion. Last year we needed to replace it for other reasons & wanted to do a fiberglass door, but they advised against it due to its exposure to the elements & heat. So, we again went with steel, had it painted a light color & it was ugly. Couldn't stand the light color. We had an automotive dealer paint it the same teal color as the first door. So far so good.

The walk-in door to our garage (S side of house), also teal painted by an auto dealer, still bubbles on the window trim & I repaint the trim about every year. We're in Nebraska. I'll be anxious to hear what others say.

My guess is the dark color and the fact that the storm door is pretty airtight. It isn't the paint that is bubbling - it is the outer steel skin being deformed. For instance, the lower right quarter of the door (looking from the outside) gets the most direct sun. This area has a noticable outward bend in the steel of an eighth to a 1/4 inch. I know black + airtight is bad but the door ought to be able to maintain its shape - we aren't talking kiln heat hear. Thanks for your thoughts...

Heat build up with storm doors that dont have an operable pane to vent are notorious on wooden doors. This is the first I've heard on steel clad doors. The heat couldn't possibly be all that bad this time of year although it's probably surprising if you was to measure the temps inbetween the two doors. I think you might be on track with the expansion/contraction but unfortunately I think your only recourse would be to hack it out with the manufacturer, or replace it with another reputable door mfgr. You might want to think about swapping out the current storm door with one that vents.