Hi, I have fixed the fluid mixing problem. But I see that the teleporting is weird. E.g. I want to move one tile up, but it first teleports 20 tiles down and then teleports 21 back up.

That is how teleporting works. It moves it out of the way. Checks if it can be placed where you want it. Then moves it to that point.

Should is also be jumping left like 20 tiles when moving it 'right'? Or should it be jumping it "into" another building (it jumped a chemplant 'into' an oil refinery, overlapping 6 out of the 9 tiles of the chemplant with the oil refinery when trying to move it one tile)?

After some more testing it looks like if something would be moved into a position where a fluid would mix wrong then it jumps almost an entire chunk in the 'backwards' direction of the attempted movement? That includes it potentially jumping 'into' another building.

Can you post a small vid. I think I know what you are talking about. The logic is not reacting if it teleports to a mixed fluid position

For note, this from the 0.17.32 patch notes seems highly relevant, Dominik (Factorio Staff) says this mod is doing something "very weird". :-)
https://forums.factorio.com/69316

I can grab a short video when I get back home for the day.

Yes, Dominik is the same person as this Dominik :)
Conclusion here at the office is that the checks do not work ideally but since it works we will leave it as is for the time being. So this jumping around is an ok workaround.

I have added some rudimentary fluidbox hax to the next version.
I might submit a factorio PR with a much better work around.