Hope you don't mind me giving my 2 cents
I have similar problem with my dog. He half sits in front and half goes to heel position
The problem is the dog anticipating a heel or another command so it's like why bother sitting right in front.
It comes down to foundation training i think. If the recall to front was really solid to begin with the problem shouldn't arise
Bart Bellons theory with the boxes or a marking board is a good route to fixing it. He'd put a box open at one side in front of him and call the dog into the box and reward there. Then when that was solid he'd remove the box.
Similarly he'd put another box to his left so to position a straight heel from the front position
I'm sure you know this technique and can search for his clips if you need it