So it's unlocked, all the linkages are connected and not bent and working as they should but the latch itself just won't release from around the striker? Sounds like the latch is just stuck. I wouldn't say it's specifically a common issue to squares, that's common with any old vehicle really. Try spraying it down with a good penetrant oil of your choice (kroil, pb blaster, WD40 if that's all you have laying around, etc), let it soak in for a while and work it around till it frees up. If it's all full of gunk you're gonna want to spray it down with brake/carb cleaner and get all the grime out of it, and spray it with lithium once it's clean and dry. Should be good to go.
If it's really rusted up, in addition to the oil try hitting it a couple times (just don't bend it), heat it a little if need be. Just treat it as any moving part that's seized up. If it's real bad and you can't clean it up at all, once you get it open you'll probably want to replace it. Even if you do free it up some and get it working, if it's that bad you're better off getting a new one because it will forever be a problem.
