Also the middle joiner and end pieces have tension controlling bolts, those could have an effect on square. If one side is super tight and the other isn’t it could make it tweaked. Try rotating the bearings 1 at a time and see if some have much more drag on the rails than others.
Not to many people seem to have this problems but when I try and replicate it it is because something isn’t “balanced” I guess is a good way to word it. I would say loosen up all the tension bolts including the 4 long ones and as you start to snug things up check the rails with a carpenters square, if don’t have one they are about $5 at the hardware store. This is how I have been checking on the new middle assemblies I have been making. If they aren’t square when it is completely loose something else is wrong, either with the rails being bent or your roller f’s are tweaked. The roller f are very strong and should hold it very square by themselves.