I can add info to this behavior:
It is not restricted to plain-text mode - also occurs in html.
The inconvenience is that it become impossible to split lines in the quote, say, to insert a reply.
The workaround is to copy the text inside the format box and then past-as-quotation to the end - then erase the entire format box.
Examining the page source when this happens shows me that the original email was formatted using a table. Splitting a line in the quote would involved adding additional tags to end the row, the columb, and the table, then restart it again.
The formatting seems to be to allow some services to add advertising, or other non-user message, to a footer below the email text.