Agreed there's no direct evidence, because we don't know the quantities and procedures used by those who fail. However, there's a lot of experimental evidence (done under controlled setting) that using too heavy dosages with the correct procedure still shows up on tests (the tests work extremely well as an upper limit), and that using safe dosages with improper timing still shows up on tests.
In fact, those are the main mechanisms for failure (the other mechanism is poor choice of PED's, but that's pretty rare among high level athletes).
I guess we differ on whether the guys who fail are more likely to screw up in dosages (my suspicion) or on timing (your suspicion). Given how straightforward and well understood the timing procedures are (the same info that Olympic athletes use to avoid detection is available to top MMA fighters), I just don't see how anyone could screw that up - there's no advantage to breaking procedure. Whereas going with heavier dosages gives an advantage, and so there is always a temptation to do so.