Don't read too much into these stills. Sometimes how a prop is used on set and how the picture is later edited in the editing room can be a process that in itself creates continuity errors, no matter how on the ball the continuity person, or the script supervisor or the prop master is. Sometimes one of these three people just screw up (or they all do). Look at this frame as evidence:
http://www.thedentedhelmet.com/reference/hd_rotj/pages/RotJ_HD_101.htm
This jet pack has no damage at all. This shot is AFTER the other shots referenced, and certainly AFTER Solo 'damaged' the JP. Now, maybe it is a special rubber version for the stunt (in which case it was made long before it was decided it needed a gaping hole) or maybe they forgot, or maybe they couldn't find the proper one (I've seen this happen to props on all kinds of sets) or any number of other practical issues could've happened. My point is, filmmaking is a messy thing no matter how much money you are throwing on screen and sometimes these are just mistakes rather than some intentionally thought-out master plan. Also, remember, when ROTJ first came out, the idea of freeze framing the film at home in high quality resolution on your TV was just not commonplace or even a consideration. All the action in this sequence happens so quickly that viewing this in the theater, its virtually impossible to see these details on a first viewing. ANow as for why the damaged JP is used in the SE, I have no idea, other than a block headed prop master or the original was not usable. Honestly they made so many mistakes in the SE Fett (shoulder armor on backwards) that it is likely they just didn't give a flying bantha poodoo about detail and continuity. Who knows.