Hi @Laurleo ...
I am sorry to hear of your frustration with the Rich Content Editor (RCE) where you were typing your discussion reply...but then it disappeared from your view. Canvas documentation does state that if you navigate away from a page where you are typing text, it stores the text in the browser cache for one day. Here's that reference for you:
How do I add and modify text in the Rich Content E... - Instructure Community - 322 (canvaslms.com)
In particular, read the section about 3/4 down the screen under "View Auto-Saved Content".
If you reload or navigate away from a page while editing content in the Rich Content Editor, Canvas automatically saves your content in the browser cache for up to one day. When you reload the page or navigate back to the page you were editing, Canvas displays a message indicating that auto-saved content exists. You can preview the content, discard the content, or load the content into the Rich Content Editor.
I know this might not be the answer you were looking for, but I hope it helps in some way.