The bug
When you adjust the fullscreen resolution slider, the highest resolution possible is on the left, but it should be on the right (like every other option, it should go from lowest res to highest res, not from highest res to lowest res.
How to reproduce
Go into a version that's 1.14.4+
Go to options and look at the FS resolution slider
Mess around with it and see how the lowest resolution is on the far right, which is inconsistent with every other option slider
Can confirm. However, this might be dependant on the OS / display, not Minecraft.