question

This is the intended behaviour.

"If there is not enough room in one of the downstream objects to receive the amount the Splitter has calculated, the amount sent to each port is reduced to keep the percentages correct."

https://docs.flexsim.com/en/22.0/Reference/3DObjects/Fluid/FluidSplitter/FluidSplitter.html

If you want to change the percentages based on the port status you can change the values in the splitter's variables tree., when you open/close a port.

The following code, for example, makes the splitter send all fluid to the second port.

Object Splitter = Model.find("FluidSplitter1");
Table percentageTable = Splitter.find(">variables/recipetable");
percentageTable[1][2] = 0;
percentageTable[2][2] = 100;

Edit: Instead you could also simply use a third thank (with a small capacity) to split the fluid between the two destinations.

fluidsplitter_1.fsm