It could also be some strange mouse wheel/scrolling driver issue, that makes it zoom out or something similar

That's very odd... Which version of Unity are you running?

I see! I'll keep it in mind if anyone else get the same issue :)

It could be that the shader is too expensive, at least if you're using PBR. Are you getting any errors on build?

Did you enable GLES in the shader settings?

Neat! You can also disable batching in the shader itself if you like :)

HSV to RGB and RGB to HSV has now been implemented in 1.18

Just tried this, and it works fine here. Let me know if it still breaks for you!
(Note that you probably want Object Pos - World Pos, your current setup is reversed)

Are the cubes flagged as static? In that case you'd get incorrect results, due to how batching works

Thanks, I'll look at this for the next update!