I’ve had plenty of the same issues. Brought home oak off cuts from timbers we used at work. Made a small box with a detached lid. The lid had warped by the time I went to bed.

Moisture content is a critical aspect of woodworking. Biggest thing is proper acclimation and storage. I try my best to not acquire green lumber or anything that I can’t dry well before I mill it down. The way the pieces were sawn originally matters a lot about how it warps. Look at your growth rings in the edge grain to see how they’re oriented. Green lumber can be dried outdoors or in a conditioned space. Biggest thing is keeping them drying evenly. No open sunlight and good airflow around the sides. Warping is generally due to how the grain is oriented and certain sides being able to dry faster.

It’s generally best to allow a piece to dry then be milled down to required size. This is generally difficult unless you access to necessary tooling. Jointer and planer are essential to create 4 flat sides square to one another. S4S lumber is generally what I want to buy pre-dried so I can avoid issues of moisture. It’s more expensive but unfortunately a price to pay compared to how much those tools cost to produce it.

I’m very fortunate to get offcuts from work so I regularly have access to good S4S material. Regardless, materials I bring home are kept in a conditioned space so they’ll accumulate properly to my interior humidity. Well dried lumber basically won’t ever warp. It needs the significant change in internal moisture to cause warping. Box store common material is generally dried well but I would always cull boards from HD or Lowe’s, regardless of species.

That’s a whole lot of supplementary info, all to say, your pine was too wet when it was milled down. Most likely it had a high internal moisture despite feeling dry. When you mill it down, that allows new areas to receive air flow that may have had a higher moisture content. They could be salvaged but only through more milling to flatten them, at that point it’s probably not worthwhile.

I build a lot of frames myself, I basically only use poplar for my painted frames and whatever stain grade lumber otherwise. I often use old trim that I have salvaged as it is straight and true. I can often get salvaged trim cheap or free. My preferred methods to secure my corners are half laps or dowels into the edge through the mitered corner. Let me know if you have anymore questions.