Related note: Checked default setup and this shouldn't be possible. Will have Fillip H. run some tests to ensure the capture chance system is working properly
From my tests, the only way this could be possible is if some item has been created that significantly boosts capture percent chance.
A lv3 monster attempting to capture a lv29 monster at 33% or above health will, with default settings, have a -68% chance to capture the monster.
A lv3 monster attempting to capture a lv29 monster at 10% or below health will, with default settings, have a -48% chance to capture the monster.
Not sure how that is possible, in the default set, I checked for example monster Rhindon, he has "Capture Percent Modifier Per Level" set to 0. Perhaps that is the bug, shouldn't that be set to something higher than 0?