Increase the maximum size of log messages
The problem was not elasticsearch indexing but the maximum message size uploaded from rsyslog udp protocol.
To fix it:
/etc/rsyslog.conf
and define
$MaxMessageSize 256k
at the first line.
I had the same problem with nxlog. Changing ShortMessageLength value in etc/nxlog/nxlog.conf fixed my issue. I hope it maybe be useful for someone.