Hello Eric,
I think in responding to questions such as this, it is important to separate the feedback relative to the style of practice from the feedback to the particular musculo-skeletal question you pose.
I’m including a link to a photo of the pose you reference which you state as “fixed firm”. Many will have to view this since it is only referenced this way in the practice you mention. Otherwise it is oft referred to as Supta Virasana. However this post isn’t about what the pose is called 
The posture is designed to open the three hip flexors - psoas, iliacus, and rectus femoris. The last of these three is unique in that it crosses two joints; the pelvis (Pelvifemoral) and the knee (Femorotibial). When these muscles are not already open AND the student uses the body weight to recline, the demand placed on the muscles is very interesting. The psoas and the iliacus are deep and very, very powerful. They’ll not go. The rectus will must assuredly yank on the patellar tendon at the knee and can cause pain.
Further, as you can see by this poor chap doing the posture, when those muscles are at their limit AND that limit prevents the pelvis from tipping posteriorly, then the pose goes into the lumbar spine.
In my view this is a pose that many students dive into too deeply and far too quickly. When that happens it is possible to stretch tendons at the origin and insertion of the muscles (one of which is the knee) AND do harm to the lower spine and sacroliliac joint.
When muscles reach their limit but the student keeps going, the tendons must act since they are the secondary support structure. So they stretch, and they are not designed to stretch. Tendons that are stretched often feel good in the moment but are excruciatingly painful within three days. Whether this is your experience or not I cannot say.
When this is added to the pre-existing conditions of knee issues and sciatica, the pose and pain make for a combination that is “ill-advised”, when safety of the student is a paramount concern. That may not be the paradigm of this student or this practice.
gordon