Poor teaching of A levels?

Sadly, it can be normal. My son had two of his sixth form teachers like this. One was the deputy head who kept missing lessons due to meetings etc. The other was the head of games who regularly missed Friday afternoon lessons to take the school team to other schools for competitions, and unfortunately DS had a triple lesson with that teacher on a Friday afternoon which was taken by a succession of cover teachers, mostly non subject ones, who just told them to do homework! It seems that the more experienced/senior teachers are the ones who do the A level lessons, but they're also the ones more likely to have other responsibilities, so more likely to miss lessons!

Not the norm here - we're (obviously, I'd say) not supposed to leave the classroom during a lesson (I mean, there's more leeway than with younger kids so you might have a chat with someone for a couple of minutes but you can't just leave a class!) and classes SHOULD be covered for absence. This is often easier said than done but even if they aren't we make sure we email our class/send a message on teams to make sure they have work to do and other staff will look in on them. Also we would usually be available to email/Teams message during the day unless at death's door. Long term absence should be covered. You're well within your rights to complain to school/college as this is poor.