In GDM only the primary external display is turned on, the other one is turned off. After log in, the other one turns on. Sometimes when I locked the screen then the (not a primary one) display stays off completely until I log out and log in again.
Worked on 3.30.0-3, broken with 3.30.1-1.
Upstream bug: https://gitlab.gnome.org/GNOME/mutter/issues/342
Proposing for Final freeze exception, this obviously ca affect first boot after install so cannot be entirely fixed with an update.
A PR for the change included in the scratch build: https://src.fedoraproject.org/rpms/mutter/pull-request/11
It'd be good to have it tested, as it hasn't been reviewed upstream yet.
I will try that on Monday when I will have an access to external displays.
mutter-3.30.1-2.fc29 has been submitted as an update to Fedora 29. https://bodhi.fedoraproject.org/updates/FEDORA-2018-c830129998
mutter-3.30.1-2.fc29 has been pushed to the Fedora 29 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2018-c830129998
Discussed during the 2018-10-15 blocker review meeting: 
The decision to classify this bug as an "AcceptedFreezeException" was made as this affects some hardware configs on first boot so it cannot be fixed with an update, it may cause inconvenience if the affected display is the main one.
mutter-3.30.1-2.fc29 has been pushed to the Fedora 29 stable repository. If problems still persist, please make note of it in this bug report.