Perhaps you can use your current multiline configuration but use a grok filter or a mutate filter's gsub option to remove "\nEND\n" and everything that follows?
There are thousands of grok examples out there, including in the plugin's documentation. But I actually think gsub is better in this case, so you can try
mutate {
gsub => ["message", "\nEND\n.*", ""]
}
but I suspect that it won't work since \n isn't properly interpreted as a newline character. Using \s should work but could lead to false positives since it matches all whitespace characters.
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