graylog natty implementation supports only limited number of timezone formats

Trying to use flex_parse_date with a timezone defined as +0300, EEST, +3 or GMT+0300 do not work in pipelines however theses are recognized here: http://natty.joestelmach.com/try.jsp#.

flex_parse_date works when using timezones defined as "Europe/Riga" or non-summertime "EET"

It would be best if more formats were supported or if documentation mentioned the supported formats.

Graylog version:
Graylog 4.0.7+c3e766c on mygraylogserver (Red Hat, Inc. 1.8.0_292 on Linux 4.18.0-240.22.1.el8_3.x86_64)