Upon the first feed the taming effectiveness immediately drops down significantly, and subsequent bait drops continue to lower effectiveness. From my observation, this stops once you can hand feed them, but by that point taming effectiveness is already in the 30% range simply from taking bait. No dinos were attacking it or aggroed to it during these taming attempts.

Also, I've separately noticed that the taming effectiveness can reset to 50% if the megachelon does take damage from something, but will continue to drop from bait feeding again afterwards.