I don't own a TS 75, but once I experienced a problem with my TS 55, and maybe a similar event occurred with your saw. After changing the blade, I thought I had properly tightened the retaining bolt, but when I went to use the saw, the blade appeared loose and wobbly. It turned out that I had not taken care to ensure the multiple parts (machined washers) were correctly lined up, and thus the blade was not properly seated and aligned when the retention bolt was cinched. I would also suspect the toe-in setting of your saw may be wrong. My TS 55 has some wobble in the hinge that joins the base to the motor unit.
This past weekend, I set up my MFT/TS 55 with a 28 tooth ATB blade (Item # 490516) to cut parts from 3/4 inch hardwood plywood for a couple wall cabinets for my garage shop, to be hung on cleats cut from more of the same plywood. Ripped the plywood to the widths I wanted using my TS 55 with a pair of joined 1400mm guide rails (one of which had never before been used). Supported the plywood on a pair of folding plastic sawhorses, using another sheet of the same 3/4" plywood as my "table" and some 3/4" PS insulation sheets to support the plywood to be cut. (I mistakenly did not buy extruded PS sheet, figuring I could use some old PS foam panels intended to go between furring strips. Big mistake! This PS foam is less rigid, less dense, and crumbs are easily broken from the edges when the Festool hose rubs against its edges.) Before making the crosscuts, I had to install ALL the components of the MFT crosscutting system, even the side stops had been removed from my MFT. I installed the fence adjacent the far side of the table (away from operator), as described in the Festool MFT manual. I set the MFT side rail stops so the guide rail had to be pulled very slightly toward my left to engage the locking tab. With the guide rail "locked down" I set the fence square to the back edge of the guide rail reference using a basic 8" blade Stanley try square (the best square I currently own) and locked it down using the Festool-supplied fence rail clamp. I placed the MFT off the end of my sawhorse "table" which I used to support the 8' long ripped stock, and used the built-in MFT fence stop to assure repeat cutoff lengths. All the cuts came out square as best I could determine using that same try square to check all four corners on each panel. They are certainly good enough for my intended use! I took care to ensure that no sawdust prevented the workpiece from contacting the fence.
One thought crossed my mind - if you set up the fence as shown in Jerry Work's excellent MFT manual, when you push the saw, the workpiece could be pulled away from the fence, thus shifting the angle.
I hope you soon figure out why you are not getting square cuts.
Dave R.
