I don't think it's that uncommon, I saw a similar but distinct problem when I was doing my A Levels. In my case teachers were pushed into teaching A Level courses that didn't match well with their degree, e.g the "wrong" science or an applied science degree which maybe didn't have much relevant content. My amateur observation was that a net outflow of older and more experienced teachers (a mix of retirements, leaving the profession altogether and changing schools) was only being partially compensated by recruitment of new teachers, who tended to be younger and come from a different sort of academic background. Not sure if this was a peculiarity of my school or more common in the industry.
[I think it's also well worth noting that most of my teachers were excellent, both knowledgeable in the subject and effective at teaching. Even those who weren't great for A Levels were popular with lower years where the material was easier and less specialised. And some of my best teachers had spend significant amounts of time in industry before taking up teaching.]