I have four computers with Radeon 7700 series cards, all running Win 10 Pro 1803 (17134.112) with Radeon 18.6.1 Adrenalin installed. None of the devices will output to over four monitors, where they once previously did. I assume that the version of 10 was identical when they last worked, but its likely its a newer version of Radeon. Two computers are fresh installs, the other two were reset.
Attempting to extend the fifth monitor within Windows Display settings comes up with an error: "The display settings could not be saved. Please try a different combination of display settings." The fifth monitor is not always the last monitor I attempt to extend to, and the port that doesn't come up isn't always the same port on different cards/computers.
All cards have six mini DP outputs. They are outputting miniDP to HDMI to an HDMI matrix which then goes to our TVs. We have tested without the matrix to verify that was not our issue.