Sunnyo,
Every knee has it own recovery timeline even on the same person, don't compare to your first TKR, you will only be frustrated.
Going down stairs is more difficult than going up, you are going to have to give this knee more healing time.
Have you tried going down stairs with your right leg first, one step at a time? This is closer to normal and will help you progress.
At 3 months you are still very very early in this yearlong+ recovery, your ROM is wonderful. Don't worry the stiffness will gradually fade, and you will soon have your life back.