You forgot one thing though. Your character already used the enhancement at least twice in a decade, which could have messed up things so badly that it couldn't reset anymore after a decade. To test this cleanly you have to only use the enhancement once, then wait a decade before trying it the second time.
Another thing to factor in is that this enhancement is made with pure Muudari in mind. Since you are just the far offspring of them with only a part Muudari blood in you, it might be that your body isn't able to handle more enhancements. If that's the case you might never be able to use it again, even after waiting decades.
I also don't think it's the forsaken doing this. Since it only plays in effect one decade after starting it, the forsaken have nothing to gain by trapping it. Certainly since they are planning on destroying the entire world anyways by unleashing the horror, so there wouldn't be anything alive after one decade.
Although probably David just didn't think anyone would actually wait an entire decade before trying it again, so there isn't anything coded in for it.