Reduced hot water pressure after closing and reopening water supply in one upstairs tub
Yes, there could be a connection between the two. In fact several things. Sometimes scale buildup (lime deposits) will occur especially in hard water areas. If the hot shutoff valve is old or your water is particularly hard you could have builup in the valve that is preventing full flow-through. Sometimes turning the main off and then on will loosen some of the scale resulting in a partial blockage. First make sure the shutoff is turned completely on. Turn the valve on and off a few times to loosen any debris. If it looks corroded I would replace it with a new quarter turn valve. The other trouble spot is in the cartridge which could be having the same issue. It could be either.
There are some tests you could run to determine if the problem is the shutoff but without knowing your setup and your knowledge level I would suggest that you replace the cartridge and if that doesn't work, replace the shutoff valve - in that order.