worst loose end in naruto?

[Uzumaki Death god]

Because it takes mastery to fully release like that, and he wasn't fully releasing his seal either. The reason Killer Bee survived extraction when Madara was revived was because he fully releases his seal when letting out the 8 tails. On the contrary, Naruto nearly died from his identical extraction because he was not yet fully capable of fully releasing Kurama. They might have been good terms at that point, but he couldn't learn that feat in one night.

All the other biju didn't have masters at the time and would therefore take their true form with little effort.

And in the boruto movie?

 

[Floydical]

And in the boruto movie?

That's true, Naruto should have had the time to master full release by that time, but no jin before Naruto had that mode, so Naruto's is unique. Maybe this means that Naruto was able to achieve the power of a full release without having to commit to it. Maybe this works better for the duo, rather than fully releasing the seal.

Either way, all that matters is the fact that Naruto is not fully releasing his seal, either in the war or in the Boruto movie.