Very interesting. I’m not sure I want to downgrade as every firmware flash carries some risk of bricking (and this firmware also contains an AMD firmware update that I suspect might be security-related), but what you’re saying still makes total sense. It would also explain why the poster in Pop!_OS AMD 7040 with 2x dell u2725qe (MST, DSC, daisy chain) didn’t experience a problem, as 3.18 was not available yet in any form then. I’m definitely going to let support know about this though.
Oh, and I actually found a workaround for the issue: If I plug the mouse into the monitor’s downstream Thunderbolt port (using a USB-C to USB-A port adapter), the mouse works and the computer doesn’t hang. That would make it appear that the problem is related to the mouse being connected to the secondary USB hub in the monitor that is internally connected to the Thunderbolt controller. Very interesting…