I had a similar “spaghetti leg” experience. I had a LTKR and was progressing with PT, walking with much straighter posture and for longer periods. About 4 months after surgery, my left leg would gradually lose sensation and be completely numb after about two blocks. It was dangerous and once while trying to “push through” the episode, my foot caught the sidewalk and I fell, luckily into a bunch of soft ice plants. I could relieve it by sitting or lying down for a bit.
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It was diagnosed and the problem was severe central spinal stenosis at L3-L4 and L4-L5. The nerves servicing the legs were pinched. When I was standing straight, the exit holes got the nerves were especially pinched. So, the spinal problem was sort of hidden by the knee problem because I wasn’t standing straight up and only took short walks. I had a minimal laminectomy and foramenectomy procedure and it was a resounding success. No more “spaghetti leg”!
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So I’d suggest asking for a neurosurgeon appointment to look at your lower spine.