and then this link shows long term side effects impacting on health and basically life. I can’t see if it says they got narcolepsy within 2 months of the vaccine.
A narcolepsy diagnosis often takes months, and it's a diagnosis that tends to come after a lot of frustrating doctor's visits and, "you're just run down - try winding down for longer before you go to bed," etc. Pandemrix (the version involved) increased the risk of narcolepsy in kids in particular, most cases started within one month, and it looks like the rest started with two months.
Modern clinical trials look for these vaguer, more ambiguous patterns - changes in people's general health/mood - with e-diaries. I don't know how they were handled by GSK back then. At any rate a side effect emerging that quickly would be seen in the trial, if it happened to someone.
The bigger problem is that it's believed it doubled the normal narcolepsy rate (from 1:100,000 in a normal year to 1:50,000 in those who took the vaccine). Now that is beyond the number of people you normally have in a trial (though similar to the numbers involved in these covid trials) and that really wouldn't be visible until after rollout.
incidentally, in places where the swine flu did cause lots of deaths, they also had a rise in narcolepsy. So, as usual there's a calculation involved, and while we might not like the idea, it's one that sometimes trades off side-effects in otherwise healthy people against the risk of catching the disease. Hence another reason for the pattern of older adults first, kids maybe never on the covid vaccine rollout.