The collar in the pic above has a rounded outer profile and flat inner. It sits squarely on the piston but can rotate in the clamp body, which has a corresponding curved inner face.
The collars can look like they are missing as they get covered in all sorts of crap, so a good disassemble and clean is recommended. Mount them to the bike with the bolts loosened (to just before pinch tight) and the piston should be able to move around laterally - I think they also have some front back (ie closer or further away from the rim) adjustability as the collar is ridged inside and “locks on” to ridges on the piston.
Been a long time since I fettled maguras but used them for about 15 years straight and their stopping power is incredible.