This is an aside to the issue, but the the mailer settings were tweaked slightly in 1.18.x to simplify things, but it looks like the docs were not updated. You should use PROTOCOL = sendmail instead of MAILER_TYPE = sendmail since MAILER_TYPE is deprecated and will eventually be removed. It won’t fix your problem though.
How are you running Gitea? If it’s a systemd service, can you post the contents of gitea.service?
I think issue is likely caused by some of the protection directives in gitea.service. Can you test running Gitea with a minimal service file like this? If this fixes issue, I would reintroduce the other directives one-by-one until issue is triggered to find out what directive caused issue.
Description=Gitea (Git with a cup of tea)
Environment=USER=gitea HOME=/var/lib/gitea GITEA_WORK_DIR=/var/lib/gitea
ExecStart=/usr/bin/gitea web -c /etc/gitea/app.ini