Saved emails disappear from Apple Mail after iOS updates

why are ios updates deleting saved mail? This is just one example…I’ve been communicating with a clinic in Costa Rica for some work for 6-8 months. We were waiting until we could make the trip for our next email. I saved all communication in a file in Apple mail, like I always do. They had to write on my gmail, as @icloud doesn’t work there. So every email had a cc to me on @icloud. Today, when needed, all of that email is just gone out of my saved files. What kind of service is this?