I have had these monitors for well over a year and have not had this problem until a few weeks ago. I have 2 BenQ XL2420Z and 1 BenQ XL2420TE, they are almost exactly the same but the Z came after the TE was discontinued. When powering on my system i get different results, most of the time my TE monitor gets capped at 720P and i cannot for the life of me figure out the problem. I initially thought the cables were the problem since the ones i had don't technically support 144hz but i replaced the displayport cables and i'm still getting the same issues. The only thing close to a solution i have been able to come up with is if i switch around the order in which the displayport cables are plugged into the graphics card it (sometimes) allows me to select all 3 monitors at 1080P at 144hz - but as soon as i restart the system my TE monitor goes back to 720P. I don't think its my graphics cards (gtx 980's in sli) because no matter what i do the issue is always with the same monitor but i don't know why this monitor would do it since its almost identical to my other monitors and this exact setup has not been an issue in the past.
If anyone knows what can be causing this or something I can try to trouble shoot I would REALLY appreciate it.
EDIT:
I also sometimes get this problem, even though my monitor is in 1080P, i cannot select 1080P under "PC" in order to set the monitor to 144Hz