There is an AMD Radeon RX 5700XT (reference card, made by Sapphire), which has two displays connected: one through HDMI, another through DP. The problem: the GPU sometimes randomly blanks the HDMI display while the computer is in use.
AMD Radeon RX 5700XT (reference card, made by Sapphire)
Dell U2718Q (two of them: one through HDMI, other through DisplayPort)
AMD Ryzen 7 3700X
Fedora Workstation 30 x86_64
LLVM 9 RC2
Mesa 19.2 RC1
Could be related to this bug: https://bugs.freedesktop.org/show_bug.cgi?id=111481
Possible workaround: plug the HDMI out and in again, or hit Super+L to make Gnome lock the screen and put both monitors to idle. Then, upon unlocking, the HDMI monitor can come back.
There is no output from amdgpu in dmesg or xorg log, or the journal when this happens.