Describe the bug
The side effect defined by the last stage (continuous application) is erased continuously whenever your fatigue does not exceed the last stage.
The issue is that is even when applied by other means (ex: potions). It only takes a few seconds and despite the time left the effect will be nullified.
This happens to the default value and any other changed (tested with slowness and hunger).

Is this bug related to another mod?
No, reproduced with only somnia.

To Reproduce
Steps to reproduce the bug:

1. Install Somnia
2. Enter survival
3. Use slowness potion while in low fatigue
4. Wait a few seconds
5. Slowness disappears

Expected behavior
The fatigue, or lack of thereof, should not affect other sources of the effect.
The time should run off normally, and probably only be affected/reset when the fatigue side effects starts it and/or when you sleep.
I would expect the non fatigued state to simple not reapply the effect, rather than directly erase it.