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What GPU/video card is installed on that desktop motherboard?

Am I correct in that you are trying to daisy chain two U2722D from the primary U2722DE? Meaning the single GPU/video card DP out port is driving three monitors at 2560x1440 60Hz?

PC GPU/video card DP out port --> DP to DP cable --> U2722DE #4 DP in port (Menu- Display- MST On)

U2722DE #6 DP out port --> DP to DP cable --> First U2722D #4 DP in port (Menu- Display- MST On)

First U2722D #5 DP out port --> DP to DP cable --> Second U2722D #4 DP in port (Menu- Display- MST Off)

Hey

this is the exact configuration, there is no dedicated GPU, that CPU has internal GPU in it(APU).. any possible reasons why this is happening?

In Windows Display settings, do all three monitors show as using  2560x1440 60Hz?

List every video out port on that motherboard.

yes 4 monitors are @ 2560x1440 60Hz

display outputs on the motherboard are:

1 x DisplayPort
2 x HDMI ports

any other ideas? it is super odd blinking randomly, it is like renegotiating display port signal from time to time, can't catch when exactly.. I am starting to think one of the monitors has electrical issue of some sort.. and I have to say D monitor variants do it more frequently then DE variant monitor

4 monitors? I thought we were working with 3 monitors =
U2722DE
U2722D
U2722D

Since the PC has three video out ports, why not forget daisy chain and connect all three monitors like this =

PC DP --> U2722DE DP to DP cable --> U2722DE DP in port
PC USB-A port --> U2722DE USB-A to USB-C cable --> U2722DE USB-C upstream port

PC HDMI --> retail HDMI to HDMI cable --> 1st U2722D HDMI in port
PC HDMI --> retail HDMI to HDMI cable --> 2nd U2722D HDMI in port