This is both in the Waze phone app and in the Map editor. When submitting a map issue, the text box is very small and not obvious. It's barely visible so it's easy to miss and only select the issue and hit send.
In the map editor, when replying to a comment about a map issue, the Done button is big a blue and the Send button is small and grey. It's very easy to spend a few minutes typing a reply then hitting Done instead of Send and therefore losing the entire message.

You should really work on your UI.