I'm wondering if you are getting new plants from the nursery, planting from purchased seed, or are you using your seeds saved from previous years?

I have been puzzled by the way my moss rose/portulaca/purslane behaved this year too. I had a hanging basket of an apricot colored bloom that did great, blooming all over in the morning, but in the afternoon it sort of closed down and looked sort of wilted. Is that what you are noticing?

I also put another plant in the ground near this container plant, and it behaved the same way. So I do not think the roots are getting too hot for the container plant, any more than they do for the soil plant. Both opened buds all over the plant, grew like crazy and became quite large. I did not have any "leggy" look, but noticed the increase in size.

It has been super humid in Mobile, feels like a sauna to be honest. I'm not sure if portulaca likes the humidity or not, but surely Atlanta is not as humid as south Alabama with Gulf moisture always present.

I do deadhead mine when they look a bit slow at blooming, and sometimes pinch off the growing tips. Like you say, they are usually strong and dependable growers. I do not think they will suffer if you pinch them back.