This type of meniscal tear occurs most often when traumatic force is generated that is strong enough to rupture the anterior cruciate ligament (ACL) inside the knee. Along with the ACL tear, a posterior lateral meniscus root (PLMR) tear such as you are describing occurs.
The standard procedure with meniscal tears is to repair them (rather than leave them alone to heal on their own or removing them). But posterior lateral meniscal root (PLMR) tears are handled a bit differently.
PLMR tears are difficult to repair. There are nerves and blood vessels close by that could be injured during a repair procedure. Because of the location of the posterior lateral meniscus, the risk of further cartilage damage or damage to the opposing bone (femoral condyle) is high just trying to get the arthroscopic instruments into the area.
For these reasons, until there is clear evidence that PLMR tears should be routinely repaired, surgeons leave them alone. PLMR repair should be done only when it is clear that leaving the PLMR tear will yield a worse result than fixing it. It sounds like that’s how your case was handled.
The knee stiffness could be coming from something else and should be checked out. Bring it to your physician’s attention at your earliest convenience. It’s probably best not to put it off indefinitely. With most orthopedic problems, early diagnosis and intervention usually yield better long-term results.