@grey j I am in a similar situation as you, though I no longer have pain in my TKR knee. I saw my surgeon at the 11 month mark AND at the 16 month mark because I can't get past 95-100 degrees, depending on the day. I also asked about an MUA both times and was told by 2 surgeons and a PA that an MUA at this point won't gain you anything. It just sets back your recovery and causes more swelling, thus making it harder to get back to where you are NOW.
Can you tell us what your knee flexion was like BEFORE the TKR? Did you had previous surgeries on that knee or limited flexion going into the TKR? That is a HUGE prediction of what your flexion will be after the TKR. People think TKR will just increase their flexion drastically, but nope---it gets rid of the pain, fixes your gait, and allows you to walk (mostly) pain-free again.
As for me, I was warned that due to my own extensive surgical history and problems with this knee, my flexion would not just miraculously be perfect or normal. It couldn't be predicted at all. I would have to wait and see.

I had a load of adhesions from a previous fiasco surgery in 2016 (A different procedure from a TKR that caused documented arthrofibrosis). THAT is why my flexion stinks, but they kind of forgot to remind me about that when my flexion had trouble coming along after the TKR. My adhesions had been flexible from 2016 (following MUA at 11 wks post-op and lots of forced PT) up to 2019 when I had the TKR. The lack of motion following my TKR (due to a severe inflammatory response to the TKR surgery) caused a slow recovery and slow return to bending---which allowed my adhesions to stiffen up again. It was just an unfortunate circumstance I had to deal with and get past.
I only mention ALLLLLL of this because there are sooooooo many different reasons that flexion can be delayed or unable to return to normal.

Time helps into the 2nd year and even 3rd year, but some of us are just oddities unfortunately. I count myself lucky to be able to walk and use my elliptical at 18 months post-op, though I miss riding a bike for exercise. I can make rotations after warming up on the elliptical, but it's not a good feeling or fun to do.
One other thing that worked for me mentally----I have to keep asking myself if I'm better off now than before my TKR, even with limited flexion. My answer is yes--less pain, able to exercise more, able to shop and go on vacation again, etc. It stinks to not be able to do stairs normally all the time or ride a bike for fun, but I am definitely better off than before. I am in NO WAY invalidating your frustrations or telling you to get over it...just wanting to throw some other things out there, in case any of it helps.
--Lisa