I have a problem where I use a Super Stimpak and my weight capacity decreases by 10 pts permanently even after the Super Stimpak wears off. Does anyone else have this issue? Is this a bug or there's something I'm not aware of regarding super stimpaks?

I've had various small glitch issues somewhat similar to that. Try saving it after the super stimpak wears off, exit game entirely and restart. It should correct itself.

Give it time. There is a second effect Super Stimpaks gives you that lasts about the same time as a normal chem dose. So, not a bug at all.

Oh ok. I looked at Fallout Wikia and they do say it gives -1 to strength but for some reason in the game it doesn't give that description. However I do remember sleeping in game and it does not wear off.

It's the debuff effect. It takes a while to wear off (3-4 minutes) even after the super stimpak has done its job.

http://fallout.wikia.com/wiki/Super_stimpak#Fallout:_New_Vegas