That’s peculiar…
When you made the configuration to graylog.config did you execute a service restart.
My apologies I’m not very good with Docker.
Graylog has a default user called Admin, is this still accessible to you? or was this changed? If it wasn’t change can you logon with "Admin and recheck the timestamp issue? For user admin, it uses timezone defined in /etc/graylog/server/server.conf file, parameter root_timezone.
The reason for this is I’m wondering if only a user created has issues or its global issue.
I have also found this, another community member had a similar issue like your yours
And this I noticed was someone with Docker. Specailly this part
sudo docker-compose down
update your graylog.env with, for example America/New_York
TZ=Etc/GMT+8 <--I added this part for you.