contains function rfc2445_fold() which does not differ between "characters" and "octets". RFC2445 section 4.1
"Lines of text SHOULD NOT be longer than 75 octets"
"That is, a long line can be split between any two characters ..."
If one UTF8 character consists of two or more octets, the function happens to put them on different lines, causing importers, such as Thunderbird/Lightning, to reject the file, because it is no valid UTF8 anymore.
The rfc2445_unfold() function of the same file ignores the different consequentently, so it is not effected.