This occurs if the tank is above the maximum portal fluid placement level, and tends to be a bit annoying when trying to move the fluid.

Not sure if this is a bug or explicitly requires additional logic. I have very little experience with modded fluids/blocks/items, so I'll need to look into other mods to see how they offer compat for scenarios like this.

What tank are you trying to put the fluid in?

I've tried PneumaticCraft tanks and IE barrels (as those are the only things I had available).