TinyMCE is removing PRE tags

If a text starts or ends with PRE tag, TinyMCE removes it, sometimes leaving the HTML broken.

E.g. start editing a new entry, type 2-3 lines, select everything and mark as preformatted (BTW, why there are multiple PRE blocks created instead of just one?). Switch to no-rich-text and see the result - some PRE tags are missing. The same happens on preview or save. PRE tags inside the text are preserved.