Since some spells (like zoom in zoom out) seem to reduce or increase the amount of mass that is present in the game without any explanation as to how this happens, some interpret that it means that the conservation of mass in "thrown out the window" in 5e.
However, another view is that the RAW is silent in how those spells work, which leaves the DM to enforce or rule out the conservation of the mass. The detailed mechanics of enlargeFor example, it could be that it extracts molecular matter from the depths of the Earth and / or from the environment or the atmosphere, or even from another plane, so that the conservation of the mass is certain. Or again, maybe not. The description of the spell does not say.
In a couple of responses on 3.5e (like this and this one), it has been softly stated that the conservation of the mass is not really respected in D & D, and the subject was addressed in the comments to an answer to a question 5e, but the The issue has not been resolved definitively, I can see that.
Is this really ambiguous according to RAW, perhaps intentionally (perhaps the authors of the rules are avoiding the excessive management of the configuration of the multi-table campaign)? Or is there a definitive RAW response to this somewhere that you have not found?