This is a common and difficult problem when simulating fluids using particles.

It's possible to simulate using a grid instead, and that computes pressure very precisely, but it has a downside of adding inaccurate viscosity.

And there are tricks that combine both approaches to balance out the errors.

Yeah agree. The objective was more to make a physics toy that would run on single core on a phone than something for actual scientific or industrial use. I could add additional iterations or do pressure projection but then there would be complaints about it being slow & choppy.

There are also some large density ratios between the materials which further increased the difficulty, and would also increase the number of pressure projection iterations on a grid. I tried to simulate buoyancy without cheating (e.g. giving different materials different acceleration to gravity)