ProblemMatchers sometimes ignores some (multi-line) warnings

[erikmd]

as shown by
https://github.com/erikmd/docker-coq-github-action-demo/actions/runs/9930595795/job/27429503898

  - File "./src/demo.v", line 17, characters 0-13:
  - Warning:
  - SSReflect: cannot obtain new equations out of
  -   (b = c)
  - Did you write an extra [] in the intro pattern?
  - [spurious-ssr-injection,ssr,default]

maybe because of the immediate newline after Warning: