First guess would be again a memory issue, but you already put higher amounts of memory, so I guess that is not the problem here (maybe you can post an excerpt of your tomcat and elasticsearch configuration - the part with the memory configuration).
Also error logs might be helpful - {tomcat_dir}/logs/catalina.out
I did not yet try with elasticsearch 6.3.x but I would not assume that that has anything to do with it (also if I understand correctly, you have the same issues already with 6.2.x)
Can you check if the catalina.out file contains any stacktraces? On normal info level there is not much logged, so you should see the error close to the end of the file, if any occured