Grok pattern fails although it is valid

Elastic Stack Logstash
2

I've even tried to replace the built-in grok patterns with regexes, but with no avail:

grok {
	match => { "message" => "\ASummary for local -------------- Succeeded: (?<succeeded>[0-9]+) (?<foo>.*) }
}

EDIT: This behaviour is very likely due to the multiline message, see Error with dissect filter. I'll do some tests and report here my findings.