Here you are the info from the manual :

PIP/PBP mode is disabled
automatically.

• PIP/PBP mode is used below 60Hz. Turn off

PIP/PBP mode if any input V frequency is more
than 60Hz automatically.

• Change Input video V frequency to 60Hz then try
to set PIP/PBP mode again.
Image is not full screen in
PBP mode.

• Set resolution to PBP window size as following:
1920x2160@60Hz in mode.
3840x1080@60Hz in mode.
3840x1080@60Hz at large window in or
mode.
1920x2160@60Hz at large window in or
mode.
1920x1080@60Hz at small window in
mode.
1920x1080@60Hz in mode.

• Check resolution in PC and OSD. Disable
scaling in graphics card if they are not the
same.

• The resolution in OSD is leftmost window.
Set the input source to leftmost window to
check resolution if the window is not leftmost
window.