Multi-Monitor Sync (MMS) is enabled. When MMS is ON, Dell Display Manager has limited control over monitors in the daisy chain beyond the first one.
Open Dell Display Manager. Check the status of the Multi-Monitor Sync function. If it is enabled (ON), DDM can only fully manage the first monitor connected to your system. Brightness and color controls for the downstream monitors will be restricted or unavailable.
To regain full individual control over each monitor in the chain, you must disable (turn OFF) the Multi-Monitor Sync function within the DDM software.