monitor Wierd Monitor fps going down

[Da Huang]

Hello, guys, I am Da Huang. I have a weird question that I have three monitors, two Dell p2715q(running at 4k 60Hz), one Asus PG279q(in the middle)(running at 2k, 165Hz). Graphic card is 1080ti(gigabyte 1080ti extreme), and they all connect by DP port. 

Then here is a question, I set closing my monitor for more than 15 mins, but every time I leave more than 60 mins, the dell monitor on the left(yes, just the one at left) will lower the fps to 30Hz, which you can not change in the setting. If I close the monitor by the power switch or just change the resolution lower and back, the fps will go back to 60hz. Also the one at right is totally fine. I am totally feel weird about that, May anybody help me with that? Thank you!