I had the same issue and managed to fix it by replacing the cables and the Thunderbod2 - USB-C adapters, which aren't compatible with MiniDP.
Once I connected both monitors with a DisplayPort -> USB-C cable, the issue was gone, and I was also able to see the 'Display Settings' and 'Detect display' buttons under System Preferences > Displays.
I hope that helps
Elro
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