That sucks!

The next time its dragging, maybe pop loose a bleeder screw and see if you get a big squirt, or just a little flow like you would expect when gravity bleeding. A big squirt would indicate there is still pressure on the system, and then at least you have a direction to chase.
If there is no big squirt, then the drag must be caused by the brake pads or the caliper.

I'm really starting to wonder what they are making the brake pads and shoe linings out of these days, and if that can be some of the brake issues that seem to be appearing in recent years.

I have all new brake shoes, springs, wheel cylinders, backing plates, and drums on my 49 truck (96 Dakota stuff). I get about 3,000 miles out of new brake shoes before I start getting a howling from them as the truck is about to stop. This it the 3rd set of shoes, the drums have been turned (yea, I had new drums turned) and those drums have been replaced. The shoes have come from different parts stores. With each replacement, I get about 3,000 miles before the noise starts again. After that, the more you drive, the worse it sounds. Reminds me of the howl some of the old big truck brakes used to make.

My issue appears to be weather related. This set of shoes does not make the noise every day, but maybe a time or two a couple times throughout the day. Hot and humid weather seems to make it worse (rarely happened through the winter). I have even pulled the master cylinder off and shortened the adjusting bolt for the booster, thinking maybe the extra under hood heat was causing the problem. That helped a little, for a while. I might pull the master and shorten the rod another 1/4 turn.