The good news is that you can rely on behaviours
I believe Ashley said in other related topics that "Every X seconds" is not affected by the framerate, it runs based on real-world time rather than ticks. Therefore I am assuming it must be running on a less accurate timer (system clock, maybe ?), or using a parallel scheduler.
The only flaw in your example, if my analysis is correct, is that you were using a less accurate method (world time) to time a more accurate method (tick based).
I'm pretty sure you should be able to replace "every x seconds" in your example with a Timer behaviour, which would then be tick based, and then get consistent results.
Basically, see "every x seconds" as a way to schedule high-level events, e.g. "every ~20 seconds spawn a new monster", not as something to be used for animation or frame times.
As for "sub tick", I can definitely see cases where that can be useful (usually used for particle effects, etc.) but that's usually very specific and hard to make generic.