Although we didn’t find out the actual biological reason why Goku loses his tail in dragon ball, we knew that it had something to do with Goku’s weakness.
Also, it much later in the series when you combined Goku having a tail with the direct exposure to the full moon Goku transform into a Great Ape.
So in that state, Goku became Berserker and was completely out of control. In the series during the finals of the 21st Tenkaichi Budokai, the World Martial Arts Tournament Goku becomes a great ape.
When Goku was in the Great Ape state he almost killed everybody until Roshi disguised as Jackie Chu fires a Kamehameha at the moon.
Roshi Kamehameha was powerful enough to destroy the moon that earth had. So for the remainder of the Dragon Ball anime and the first few volumes of the manga were all without a moon.
It’s not until Goku gets to the Heavenly realm for the first time and meets Kami also known as God. That was the only time we got some clarification about what happened.
When DiD Goku Lose His Tail
Although we don’t what happens off-screen, it is confirmed that Kami did indeed remove Goku’s tail from the root.
This was to prevent Goku from ever-growing it back and the reason why is because the moon needed to be recreated by Kami.
Kami made sure that the moon would stay there safely and that Goku would no longer be able to transform into what is known as the Great Ape mode.
So Kami decided it would be best to remove the moon even though, he is the God of Earth. The Moon is a satellite of Earth and thus, as a result, he has the authority and the ability to create a new moon.
This is why you never see Goku as an adult with a tail because it was removed from the roots.
All we can do is speculate but we know for sure that it was commie who removes Goku’s tail while Goku was training with him for three years before the 23rd tournament.
That was when Goku is set to match up against The Reincarnation of Piccolo AKA Piccolo Jr. as he had instructed him to do.
With that said this I hope this helped you with your question about When Did Goku Lose His Tail In Dragon Ball.