Here's a 2019 article, "Study suggests how measles depletes body’s immune memory"[0] which (despite the title) seems to quantify the percentage of antibody arsenal that is lost after a measles infection. Sadly, I haven't had luck finding resources to answer "by what mechanism does measles cause antibody memory to be lost?", but I've got a good thing to ponder for the weekend. Thanks. :)
[0] https://news.harvard.edu/gazette/story/2019/10/how-measles-w...