Even if it was initially a bug, Hello Games now treat it as an official mechanic. One of the patch notes during the Expedition update (3.37, I believe) calls out "fixed an animation bug when melee jetback boosting." It seems HG has been well aware of this for many years and explicitly don't intend to remove it.

How the mechanic/glitch functions has actually changed over time, which indicates to me that they've deliberately meddled with it for balance purposes. It used to be, for instance, that if you melee-boosted into water from land or the air you would keep your initial forward velocity in the water and could "coast" at that speed as long as you kept holding forward and did not hit anything... even without jetpack input. This is no longer the case, which is a bummer because it made traversing large stretches of water less tedious. It also completely obviated the need for a Nautilus unless you were planning on diving, though, which is probably why it was tweaked. IIRC that change happened with the Abyss update, or whenever it was they changed the water effects to allow for the large waves during storms.