DIAGNOSIS The shoe pivot lock isn't fully engaged or the locking mechanism is damaged. The shoe is designed to pivot and be locked at angles.
FIX Tighten the pivot lock lever. Locate the large, knurled plastic lever on the right side of the shoe. Loosen it fully, set the shoe to 0 degrees (using the scale), then tighten the lever clockwise as firmly as you can by hand. There is no separate tool needed.
IF THAT DIDN'T WORK The serrated metal washer or the pivot bolt underneath the lever may be stripped or worn. You'll need to remove the shoe (single screw at the front) to inspect and potentially replace these parts.
HEADS UP Overtightening the front shoe screw (the one that allows it to slide forward/back) can also bind and distort the shoe, making it seem loose. That screw should be snug but not cranked down.