Reported at: https://forum.kopano.io/topic/182
Seems as multi-byte characters are "cut in the middle" when truncating (German). This probably causes iOS to break on wbxml decoding and to loop (and ignore) the email.
Preliminary tests using mb_strimwidth() in Utils::Utf8_truncate() have fixed the issue for Richard.
Still, the truncated string presented a "?" at the end using mb_strimwidth().