As painless does not support \n, as per https://www.elastic.co/guide/en/elasticsearch/painless/current/painless-literals.html
you have to get ASCII newline 0x0a, cast to char and then cast to string https://www.elastic.co/guide/en/elasticsearch/painless/current/painless-casting.html#character-string-casting
So to split your string in Painless on newline you have to
_source['message'].splitOnToken((String)(char)0x0a);
And yes that sounds retarted.