What does the surgeon say? Have you had an imging done to see what is going on?

3 months seems way too soon for doubles tennis. Very surprised your surgeon would suggest that at 3 months.
I had a MUA at 14 weeks. My ROM became very limited due to scar tissue. The MUA loosened my knee up considerably. Started daily PT the night of the MUA along with a "Dyna Splint" for extension. I am currently at the end of the second week after mua and my gait is considerably better as is my ROM.
I too am looking forward to getting back to doubles tennis, however, I don't think it will be anytime soon
Does your surgeon say what could be causing the buckling?

I was told that, most likely, i would be able to sprint in the Sr.Games in a year after my May 2023 replacement. Now, 4 procedures later, i have trouble walking as a result of the severe Arthrofibrosis, swelling, limited ROM and pain. They fail ti mention up front that 6-10% of those having knee replacements will end up with Arthrofibrosis (excessive scar tissue buildup).
It is caused by an overactive Immune system. As I understand it, The Hospital For Special Surgery has developed an immunological test that will identity those who will react poorly in advance of surgery do that they can avoid it. I wish there had been such a test ling ago, as I would never have had the replacement. My next option is a revision to a hinged knee, in which they remove all tendons, which are an area where binding scar tissue accumulates. There are no guarantees that come along with it, so i am on the fence.