This has happened to me on occasion. Never did track down the reason. Maybe fat-fingered the wrong set of keys that sent into the bit bucket.
I now use a text editor with spell check to compose a response. I use the same file name to avoid a bunch of text files clogged up the drive. Then cut and past when ready.
It is also good if there is a problem while composing, system freeze, power loss, etc. You can keep saving every so often.