It’s happened to me, although I can’t say “it’s not my fault”. That is the reason, any lengthy post I do in a note first, so I don’t lose anything.
This is good advice. Use Notepad (Windows) or whatever comes with your operating system to compose. Copy and paste the text into your reply box.
You could even use this if you like:
https://onlinenotepad.org/notepadAs to why it occasionally happens ....