Hi
I understood what was the problem.
cron has generated a segmentation fault in the middle of processing and has ended.
The occurrence is in the processing of core_text::encode_mimeheader called by the encodeHeader function of moodle_phpmailer.php.
iconv hangs when converting from UTF-8 to ASCII in core_text::convert.
(I am using Japanese.)
The environment in which the problem occurred is as follows.
Moodle 3.5+
PHP 7.2.1
iconv 1.14
When cron works correctly, the status is "complete".
I hope that someone's help.