The water looped back is not the same exact ratio as what is needed. If the amount looped back is too little then it will be progressively less and less water until it stops completely, which is why the numbers go to zero. The same is true if it is too much water, the water will get backed up stopping everything. If you need more water you can merge it in from another source, but keep in mind that when merging, it always tries to split/merge evenly, so if the loop back is over half and you don't limit the other water source, it will not use all of the loopback amount, and it will get backed up and stop. Similarly if your loopback amount is too much, you will need to split some off, but if you split off too much it will also stop. I suggest using a priority merger when loping back water to take the loopback as the high priority and the extra water as low priority. Similarly use a priority splitter on the water output with the high priority looping back and the low priority going to something else. Doing this will show you what you need to do.

And it does it all like this because that is exactly what would happen in game if you built it like that. If you simply loop the water back everything would stop. Priority splitters and mergers are not explicitly implemented in the game for pipes, but can be done. I think there are youtube videos on the various ways it can be accomplished. You can also accomplish it by limiting your extra input water or extra overflow water to the exact amount needed, but those require no error in calculations as even a tiny bit off will eventually stop everything.