So i was lvling my THM with a shield, rank 9-20 and then i hit fatigue. i was getting 200-300sp for shield and THM. So then i switched to my CON. rank 9 and started leveling that. At rank 9 you can get 400-600 sp per kill and my shield was getting 10% less of that because of fatigue. So it seems It doesn't matter what rank your shield is. The sp you get for your shield goes off of your current class rank. GLD,THM or CON. I ended up getting 0 sp for shield because of the fatigue. but it capped out at 24 starting from rank 9.
I'm not sure if anyone else noticed this or not but if you want to get your shield rank up level GLD,THM and CON. You will rank up your shield fast. Or it could be a glitch that SE missed.