(SOLVED) Can someone explain to me, what is happening?

What is happening indeed, lol.

I'm guessing the red liquid is denser than the water, so it sinks. The reason for the rendering issue is due to them both being transparent blocks. If you notice, when you have multiple transparent blocks touching (like glass) you will only see the side that has air touching it. Any side that is against another solid block does not render, as it would not normally be seen, so in order to save on computer processing, the game does not render it.

I've never tried personally, but try setting it to -1 (make the liquid considered to be a gas)

I'm not sure, lol. That is a possibility. 

As a general I hate the liquid creation, lol. I made a custom lava and you can swim through it like water. No matter what I do, it just doesn't work.

No, I haven't, which I feel pretty dumb for. I could probably do an override velocity by get velocity/2 or something. IDK why I'm only now thinking about this, lol. My discord is stitch147 or possibly stitch147#5100 if it asks.

@joao.mp14 did you use mcreators feature tab or custom liquid tab to generate the liquid? what's the generation settings?

@Stitch147 to make your water like lava you'll need to setup your liquid, save it, and go to the folder with it in it, copy and rename it to a new liquid on file and inside. then remove or add flags you want and save it. go back into mcreator and the new liquid should appear. 

yes you can use features tab to create lakes and etc. 

also can you just screenshot each page. it's a lot more helpful. Use imgur or discord to upload pics, then hit copy image, not address, don't use mcreators forum image thing, just hit paste, super duper easy.