I've been having the same issue.
Dell monitors have this weird features such as MST and DP 1.2.
Turn it off and see if it works
see full list:
https://www.dell.com/community/Monitors/Single-monitor-on-a-PC-MST-Off-DisplayPort-1-2-Disable/td-p/6131242
(you turn this off in the screen options that is built in, not on the mac)
Amenefus
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