It's might not just feel like 60Hz, it could really be reduced to 60Hz.
When you have different monitor refresh rates and you open any program/window with GPU accelerated content (like youtube in web browser) on the lower refresh rate monitor, the fullscreen game on the 144Hz will drop to 60Hz.
Limit fps to 120 (if you want a smooth 60fps stream and play with higher fps than 60) and close windows/programs on the second monitor. You might also need to turn off Gsync/Freesync, but it depends (just test it).