It's a tough one because I cannot say with 100% certainty that AI is the issue. Anxiety is definitely a possibility in some cases, but not all; perhaps thinking time might be a factor, or even just good old copying and then running the work through a paraphraser. The large amount of absenses also means it was hard to benchmark those students based on class assessment (yes, we are always tracking how you are doing in class, not tp judge you, but just in case you need some extra help!).

However, AI is a strong contender since the "open book" part didn't include the textbook, it allowed the students to take a booklet into the exams with their own notes (including fully worked examples). They scored low because they didn't understand their own notes, and after reviewing the notes they brought in (all word perfect), it was clear they did not understand the subject.

That sounds like AI. If you do your homework then even sitting in a regular exam you should score better than 20%. This exam being open book, it sounds like they were unfamiliar with the textbook and could not find answers fast enough.

Not the previous poster. I taught an introduction to programming unit for a few semesters. The unit was almost entirely portfolio based ie all done in class or at home.

The unit had two litmus tests under exam like conditions, on paper in class. We're talking the week 10 test had complexity equal to week 5 or 6. Approximately 15-20% of the cohort failed this test, which if they were up to date with class work effectively proved they cheated. They'd be submitting course work of little 2d games then on paper be unable to "with a loop, print all the odd numbers from 1 to 20"