The pendulum FPS isn't super accurate and a bit finicky to set at times. Stock range 40-75Hz, set 20-75Hz and you will find out that 38Hz is not working correctly, so you set lower range to 40, check again if you can push FPS down to 1 from say 45fps without issues and tearing or stutter. Set 39, and test again, set 38 and test, set 37 and test, ... until you get and issue say at 38fps, so you back up by 1Hz. Because you're upper limit may not be as high as required the LFC may not kick in, if your monitor supports above 75Hz then raising max will definitely help.
For me I can see it that LFC works because the overdrive artifact starts to look as if I have double the refresh rate. And I get no tearing or stutter all the way to 1fps, though pendulum demo has minimum 20fps, you have to test lower in games with ingame limiter or RTSS.
With my GPU and monitor LFC turned ON at minimum range + 1, so if I set 36Hz it would run LFC at 37Hz = 74Hz, well probably anywhere below 38Hz, so 37.999Hz, since the reported range max is 76Hz.
When you set 35-75Hz and cannot get issue free 37 and 38..., LFC will turn ON at 36.999Hz = <37Hz = below 37Hz, as such it will run 75-39Hz fine, 38+37 with issue and 36, 35, ... and below will be doubled to 72, 70, ...
You should thus set 37-75Hz (or what ever other max your monitor supports be it by default or OCed) and it will run: 75-39Hz fine and anything below 39 should get doubled, the problem is 38*2 = 76Hz, and 39*2=78Hz, so if your monitor can't run 78Hz it may not work unless it locks itself to 75Hz and you raise the upper range limit to 78Hz.
For me it works without having to change upper limit because 36-76Hz ==> LFC at <38Hz ==> 38*2 = 76Hz and everything is OK. Maybe you can set the upper range to like you did to 80Hz, maybe it will not run 76Hz+ issue free but you can tell it not to with Vsync or FastSync to limit it to 75Hz, kind of trick the LFC to start is all you really need. On my monitor raising the upper range had no effect as far as I remember, it would go out of sync above 75Hz, it would work but with a tear.
It's what I did, well I did 1-144Hz and checked for when tearing start on upper end and when issues starts on lower end. You can probably set 37-78Hz or 37-80Hz and get tear free 1-75Hz experience with LFC under 39Hz.