For me, Fett’s armor differences themselves are par for the course from a production standpoint. His armor has been in flux right from the beginning. In the details, the armor was quite a bit different even from TESB to ROTJ. The weathering, the color of his cape, his gauntlets, and even his helmet were all different, not to mention the paint schemes and color palatte across the board are different as well. A more striking, but not exclusive, example of color change is his jetpack. It went from being almost totally solid green in TESB to bright blue, white, yellow and red in ROTJ. So the lack of attention to continuity detail from a production standpoint is nothing new at all with Fett. It was “close enough” and that was apparently deemed “good enough”. Your average Joe Blow likely never noticed.
In my own head canon, with all the TESB/ROTJ costume differences adding up, I always thought they could plausibly say canonically that the guy in ROTJ wasn’t actually Fett at all, if they really wanted to go that retcon route. Maybe it was just a flirtatious, cocky buffoon (Jodo Kast?), macking on Jabba’s harem and carelessly going toe to toe with a Jedi and getting himself tossed into the Sarlacc in the process. That guy is dead but the real Boba Fett, the guy seen in TESB, was still out there somewhere. Obviously they didn’t go that route but the costume differences were plentiful enough that they could have, imo…