To correct this, you have to edit the aspect ratio values in the guisettings.xml file for all required resolutions. I changed only 1920x1080 modes as I'm using only them.
Unfortunately changing the settings directly in the guisettings.xml file did not work for me, as it won't save the values i put in there and will always revert back to the wrong settings once a video is played or after a reboot.
Also tried overriding these values with the advancedsettings.xml file but still no luck there, it won't override the guisettings.xml in this case.
The only way to make it work here was to play a video file of every refresh rate which gives me the wrong aspect ratio, bring the OSD, go to video settings then "video calibration".
From there i was able to change the wrong values of "pixel ratio", most of them where at 1.008 instead of 1.000.
Doing it this way did change and saved these values in the guisettings.xml file, not to 1.000000 as i could only change up to 3 numbers after the decimal point but it is still very close and now these videos plays at 1920x1080 instead of 1904x1080 so all is good now.
Thanks for sharing your findings
I guess you are having more luck than we did, on what kind of hardware you have LibreELEC installed on?
Here is intel 4690s + Nvidia GTX 1070