Tested it out with Urgot in the practice tool against a Brand. Even with his RoA fully stacked, your ult barely heals you for any damage because he needs to be below 25% max health, and it only deals his current health in true damage when executing. I tried the same with Pyke and got the same results, though with even less healing because of Pyke's R being an AoE and thus gaining reduced healing.
The way these types of executes work is that they deal the champion's HP in true damage first, then, if they're still alive, they start to deal more and more true damage until they die. If, for instance, the enemy has a Janna shield on them, the first phase would leave them with only the amount of health the shield had. Then, it would start to deal larger and larger amounts of true damage in order to finish the job. It doesn't start at dealing 9999999 damage, it only increases to that amount if the target doesn't die quickly. Since practice dummies don't die, and can take an infinite amount of damage, you immediately deal the maximum damage an execute can deal, and heal for a small percentage of a very very large number.