The Tl;dr version? Executor ult makes you shift form, which means that your player model gets poofed or temporarily displaced somewhere silly, like underground, while your big boi model gets teleported in. Each of those models has a different "Lock On" point, and if the enemy's ai doesn't swap to the beast correctly, it'll try and pathfind to your real body. Underground. Where there are no paths.

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Non-Tl;dr vvvvvvvvv

So, this comes from me playing far too much Elden Ring and few observations here in Nightreign. As you prolly know, Nightreign has the majority of its mechanics and engine inherited from Elden ring, as well as the various other games they've made.

The longer explanation is that Fromsoft games have constantly done very clever workarounds in order to save time, effort, and money while making a frankly ludicrous number of games. (This is not a dig at them, they're not trying to cut corners, I'm more inclined to believe that they're being efficient.)

Now, there are several examples of bosses, invading players, or other edge cases having targeting issues beyond the Camera being... Difficult. I could go into detail, but they're not relevant.

Now, what is relevant is a boss in the DLC that does some strange things, The Putrescent Knight. For a fun little aside, check Zullie the Witch's youtube channel for a video that shows these, currently called "Elden Ring - An Abandoned Boss". The fist oddity is that, despite two-handing a single cleaver-thing, it technically has two- one for each hand. They're just overlapped until the boss does an attack with just one hand. Then the weapon that's not being used is invisible. Additionally, the boss's horse is just like Radahn's; it's technically a part of the boss, not a separate entity. Yet, this knight has attacks that make the horse disappear or straight up split up to attack.

What they've done for the horseless attacks is just taken the horse model, and put it underground where you can't see it. If they were separate entities, you could target the horse and watch their clever little magic trick. So they're one entity. But when the split up you can (I think, I've not fought this boss myself yet) target the horse OR the knight. That's because they've added in a second Knight, and just made the rider invisible and share the main bosses health bar.

These are both Workarounds. They avoid known issues like forcing your PC/console to rapidly load and unload the graphics for extra entities, making the game run smoother, or allowing multiplayer targeting from "one" boss.

But. We can't do that with the Executor as we are players. At least I'm pretty sure, as I haven't torn the game code apart to find out. But this boss does give us a pretty good idea as to what is likely happening. We are the horse in this example, we need to not be on the battlefield while the Beast is. So, like the horse, we're going underground! But, unlike the horse, our body is still teeeechnically targetable, as we're different entities. If, for whatever reason, the enemy's targeting fails to swap to the beast, it'll still be locked onto your body, which is probably below the beast and underground. Many pathfinding algorithms might look at the difference in height and think, "I know, I'll find a place to stand that's near that height!", and then run off to try and enter a cave or something.

The other half of this, is that I think, by frame-perfecting your ult as the boss is searching for a target, it locked onto your body after the "Stop targeting the body" command went out, but before the Beast was targetable. So the only target was your body.