> Another example is a slightly toned-down account, compared to other versions, of Merlin’s sexual encounter with the enchantress Viviane, better known to Malory readers as the Lady of the Lake.
It's actually not that surprising. Writing media like paper was extremely expensive. Artists too recycled canvasses.
Fun fact: in Italy before the Romans there were people who spoke a language called Etruscan. It remains untranslated to this day because there are so few written samples of it, usually confined the markings on tombs and such. This was U suspect exacerbated by the Gallic sacking of Rome around 387 BC [1]. It's for this reason that the very early history of Rome is fuzzy other than apocryphal tales such as Romulus and Remus.
But the longest Etruscan writing sample was a recycled linen text used as a mummy wrapping in Ptolemaic Egypt [2].
Cheap paper is still a relatively modern invention.
Rareness is relative--there weren't a lot of books, but there weren't a lot of people who could afford or read them, either. So an old book was just an old book. Printing was increasing the number and affordability of books, so saving an old manuscript may not have seemed a high priority. Possibly the rest of it was already lost/damaged.
Or perhaps it was scrapped because the scribe had purposefully censored the sexy bits of the original story, so it was known to be less valuable. After all, Han shot first!
Watery tart!