Solved!
Noah, the Proxy option was not the problem… I was using Original/Optimized. But when I tried using Proxy, as a test, the same issue happened, only the image quality was lower.
I solved it, I think it was due to the fact that each embedded project had a shape in the top layer, a black solid with a rectangular negative mask, serving as a letterbox. For some reason, when exporting, this was taken into account as some crop intention, I don’t know…
So I solved by placing the letterbox solid within the master project, over the other embedded projects. I thjink this resolved it, I am waiting to see the results exported, but I’m pretty sure this solved it.
Thanks in any case!