Originally the pre-production Su-27s had straight tips. However they'd suffer from flutter and actually caused some crashes. They tried fixing the problem with weights, but it didn't help much and the tips had to be cut off at an angle, which did fix it. Coincidentally the pre-production F-15s also had that problem on their wings, and McDonnell applied the same solution, cutting off the edge at an angle.

One of the changes that the Su-27M had over the regular Su-27 is the rudder. It was taller, and housed a small fuel tank. Why are the tips straight? To be honest I don't know, but given they are I assume they don't have the flutter issue of the early Flanker rudder. They are structurally different, and the higher aspect ratio may have fixed the problem on its own. Furthermore, the rear fuselage on the Su-27M was strenghened, however I don't know if it affected the rudder, from what I know all it did was increase max permitted G load from 8 to 9.