Hello, writing a paragraph (i.e. HELLO), positioning the cursor after the H and pressing backsape the H is deleted but the cursor is positioned after the O (end of the document) and not before the E as expected. You can experiment a similar behavior even with more paragraphs each time you delete the first character of the paragrph the cursor is positioned at the end of the whole document. The bug is present also on the prose-mirror home page.
A similar bug is present if you position the cursor before the H and press the canc button. In practice each time you delete the first character of a paragraph the cursor is positioned at the end of the document.