Solved the problem. I read the Xrandr page on Arch wiki and found the solution there.
The solution is pretty simple, I just had to delete the monitor.xml file on ~/.config/ folder, reboot, and after that xrandr started to work as expected.
I had Ubuntu Gnome 16.04.2 installed and I managed to get my laptop to work with my external QHD monitor through HDMI. My laptop have a 3rd gen core i5 and an AMD HD8850m.
To create and use a custom resolution I used UMC to get the correct modeline for 2560x1440 55hz. I used the following command on terminal
xrandr --newmode "2560x1440x55" 221.000000 2560 2608 2640 2720 1440 1443 1447 1478 +HSync -VSync xrandr --addmode HDMI1 "2560x1440x55" xrandr --output HDMI1 --mode "2560x1440x55"
With that, this resolution started to work on Ubuntu without any problem. I tried the same thing on Antergos, but when I choose the output, the screen blinks, return and ask if I want to keep the current configuration, but it actually is still working on a lower resolution.
How can I fix this problem?