Teachers who are sadly in this position know that it's a shit long term strategy. School leaders know it's a shit long term strategy. Of course it is. But they get forced into it because of short term pressures. If you have a couple of members of your department off on long term sick, with uneven supply cover as your only option (because there is a teaching staffing crisis) obviously you rejig the timetable to put the permanent teachers in with the exam groups. If you have to split a class between two teachers because a significant number of your staff work part time, would you do that to a year 8 class or a year 11 class? If you are working yourself to the point of illness, as many teachers are, which set of books do you mark in detail - the year 7 or the year 13?
I've neglected ks3 in the past. I had gcse and a level groups with coursework and exam practice and I had a finite amount of time. I've been the head of department trying to put together a workable timetable, who put the sixth form classes in first, then gcse, and then fit the ks3 around that. I managed to avoid any three way splits, but most ks3 classes ended up split between two teachers. I'm currently working as a supply teacher in an ofsted "good" school, and I'm the fourth or fifth teacher this year for a lot of my groups - including year 10. The year 8 geography class I hear rioting next door are on their 6th teacher so far this year. When they get to go see they I'll be behind - so the school will exert pressure on the poor sod who has them on their timetable to pull up their results by a huge amount in a short space of time, which will mean they will be taking up more than their fair share of the teachers time, which will end up coming away from their ks3 classes. And round it goes...
The system is broken. Teachers are broken. Schools hide this stuff from everyone they can because it's shit. But it is happening, and it isn't rare. It's pretty normal.