It seems to me that the AI-generated summaries often contain new vocabulary. If it’s not part of the LLM prompt yet, I suggest to add a request to try to use words and grammar structures that already appear in the summarized text, and possibly words and grammar structures that it is certain are lower level in terms of learning difficulty (i.e. being allowed to use beginner-level vocabulary and grammar in a summary of what is obviously an advanced level text). Maybe something like this:
“Create a summary in simple {{target language}} for language learners and people with comprehension difficulties. Avoid introducing new advanced vocabulary; instead, try to use words from the text or much more frequent vocabulary, and grammar structures appearing in the text or much simpler grammar structures that are more likely to be known to a learner.”