It was ever thus; two of my O levels were affected like this; one of my DC had two GCSEs affected like this and, for another DC, one of their A levels.
All you can do, at this stage, is be positive and emphasise that the school has acknowledged the issue and is putting a plan in place and that at least recently taught material will be fresh in the mind; encourage your DD to have an overall revision plan that strikes a balance, as best as can be achieved, between the catch-up subject and other subjects, esp key ones like Maths and English Lang.
Because of my experience at O level, I encouraged my DC to familiarise themselves with the syllabus and their progress against it, so they could feel a bit more in control and could (like me) decide whether to take mitigating action. At least these days, specifications and self-study materials are more readily available.
Obviously none of the above is ideal and, with retention and recruitment issues, may get worse. I agree that schools can be late in declaring there is a problem; in a backdrop where there is a lot to teach, a problem in, say Autumn Y11, may well escalate things very quickly without elapse time to address it.
My school did nothing to address the syllabus shortfall for me; it was down to me to sort it. At least, for my DC, attempts were made to assist with catch-up and that sometimes meant other teachers stepping in with extra lessons, for which I was grateful.