If I type in a reply then decide not to submit, I can "erase" it but if I come back to it later and click any "reply" it's back.

It saves "drafts" of your responses that you don't send. It's a feature.

The feature sucks. If you make a typing error and hit "undo", instead of undoing the last bit of typing, it undoes everything back to the systems last mini-save. I've lost entire paragraphs until I learned to just not use undo anymore.

When it does that, can't you just hit "redo" to put the paragraph back?

Yep, just tested it. You can just hit redo and it comes right back.