I guess I found the cause: The multiline string is rendered correctly only if all the lines start at the same column than the first one:
Wrong
after_success:
- >
run_test.sh
arg
--flag-1
--flag-2
Good
after_success:
- >
run_test.sh
arg
--flag-1
--flag-2
I don’t know if this is part of the YAML spec. In other projects I used several levels of indentation without problems in multiline strings.