If you’re air handler in garage and master bedroom way at back I suspect low air flow and conditioned air warmer do to length of duct leading back there. I had same problem and tried several things with little success. The air just hotter due to distance from air handler and low air flow due to smaller ducting.
I used mini anemometer
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And temperature gauge trying to adjust air flow opens in living room to more flow to back bedroom. I also tried speed air handler fan up with little success. Good luck, if find remedy without adding mini split in back bedroom lets us know?