I have had the same issue (display timeout is max 30sec, any higher value has no effect) with a freshly installed FP Open OS and I was able to fix it (took me some time to dive into that … bug?).
What happes is that the setting “screen_off_timeout” (thats the variable you can modify using android settings dialog) is set correctly to e.g. 1800000 (that is 30min) BUT there is another value in androids config-datbase called “sleep_timeout”. That “sleep_timeout” is afaik “-1” on most androids which means unlimited if I remember right. Unfortunately FP Open OS seems to have a hardcoded default here which is set to 30000 (30sec) and there is NO option in the settings-app to change that.
If you look at that code-example, you see that this value will always win if you set something higher than 30sec in your settings app (so one cant go higher then 30sec in FP Open OS):
https://android.googlesource.com/platform/frameworks/base/+/05af6ad^!/
if (sleepTimeout >= 0) { timeout = Math.min(timeout, sleepTimeout);
My fix is: As root (!) you modify the sleep_timeout in the config-database to “-1” by entering that in a shell directly on your phone (as root):
settings put secure sleep_timeout -1
NOTE: Modifying the config-db directly is dangerous and can break your phone - but I assume a FP Open OS user is aware of that. 
PS: That app might help to read out ALL settings (only read, no write):
https://play.google.com/store/apps/details?id=de.SmartDyne.Free.DisplaySystemSettings&hl=de