A few thematic contradictions have come from the two liquids being functionally the same. In general, this limits the amount of depth the part can have, since if lava is ever associated with fire, fire damage, heat (for thawing), or the burning status in any way, it breaks immersion in themes that treat it as poison. If a poison status is ever added to the game, or poison is expanded in some other way, well it can't be because in some themes it's lava instead.
In specific, a few confirmed features such as Water pots/bombs, and potentially hazard levels, as well as existing features like ice blocks, have so far been unclear as to how they should interact with lava and poison due to their thematic association. In order to avoid lava and poison being left behind in terms of complexity and consistency, I feel they should be separated and have each theme gain the liquid it's missing. Lava could be the same color in all themes (to reduce extra work and because lava is more or less a real thing), and it would ofc be up to the art team if any particular theme should get a new poison color (different flavors of liquid poison make sense since it's a fantasy element anyway).
In conclusion, I just think it makes sense and shouldn't be too much work. It would open up the potential for fun and unique interactions with each liquid type, and answer a lot of questions people have been having since the start of the game. 