same issue while viewing output via local file 
<134>haproxy[14880]: 192.134.152.190:25145 [18/Feb/2022:14:59:29.400] web-service-https~ Internet-prod/back1.local.intra 0/0/0/17/48 200 89837 - - ---- 272/272/1/1/0 0/0 {www.toto.fr} \"GET /core/assets/vendor/jquery/jquery.min.js?v=3.5.1 HTTP/1.1\"
Version with no custom patterns ...
%{IP:client_ip}:%{INT:client_port} \[%{HAPROXYDATE:accept_date}\] %{NOTSPACE:frontend_name} %{NOTSPACE:backend_name}/%{NOTSPACE:server_name} %{INT:time_request}/%{INT:time_queue}/%{INT:time_backend_connect}/%{INT:time_backend_response}/%{NOTSPACE:time_duration} %{INT:http_status_code} %{NOTSPACE:bytes_read} %{DATA:captured_request_cookie} %{DATA:captured_response_cookie} %{NOTSPACE:termination_state} %{INT:actconn}/%{INT:feconn}/%{INT:beconn}/%{INT:srvconn}/%{NOTSPACE:retries} %{INT:srv_queue}/%{INT:backend_queue} \{%{HAPROXYCAPTUREDREQUESTHEADERS}\} \\"%{WORD:Method} %{URIPATHPARAM:request} HTTP/%{NUMBER:http_version}\\"
This working under ALL grok debugger (elk, online etc) , but not for logstatsh.
i do not understand logstach grok ...