Invisible highlight color on Mac Mini M1 with external monitor

On my Mini M1 (Big Sur 11.2.3) I can only see highlight color in a non-active window when I turn on night shift. When I connect the monitor to an Intel MacBook AIr (also 11.2.3) I don't have the same problem.

This happens only with non-Apple applications.

In the samples below I'm showing a TextEdit window and a Postbox (mail client) window. I ran the test on my Mini in Safe Boot to avoid problems with third-party software,

I'm using a Dell U2711 UltraSharp monitor (2011) connected via USB-C (mini) to DisplayPort using a Plugable cable (I also tested with a HDMi-HDMI connection with the same result). I must mention that I also get an on-screen message when I log in "Increase Display Contrast On". No clue where this comes from.

In the first sample below I cannot see any highlight color in the window behind TextEdit UNLESS I turn on Night Shift (second sample).

NOTE: if you don't have the problem I describe you probably will see no/limited differences between the samples!

Night Shift turned on:

Intel MacBook using same monitor and connection (USB-C to DisplayPort). The highlight color in the active window is bright red and in the in-active window dimmed red 9as it should be).