Create a note that has a null first line and a non-null second line. In other words, start a new note by first hitting the Enter key and then typing in some text. The text will be on the second line of the note. Make sure the note is well and truly saved.
Open the note second time. Without moving the cursor at all, type some text on the first line, not followed by hitting the Enter key. Unslide the note. The HTML junk will have appeared. Or click the Checkmark icon and Unslide the note. The HTML junk will have appeared.
It seems likely to me that this is not a bug in RM8 itself, but rather is a bug in the library that RM8 is using for the text editor.