I am also going to chime in, as my posted bug report is a duplicate to this.
https://forums.factorio.com/134911

For me it is very unintuitive, that for the machines, that allow flow through, fluids can flow back out of the machine, and adding a tank even worsens this issue.
From my point of view ...

Setup:
Empty tank, Foundry that creates holmium plates, and a electromagnetic plant also requiring holmium solution.

Problem Description:
If you slowly output holmium solution into this system (e.g. using a chemical plant), then the foundry will take all the holmium solution.

Problem:
The ...

Ok, I guess that's fair :D

However I still think that this is unintuitive behavior, as you can even manage the expansion group size. And if I am able to manage that as a player I would expect that to have a direct effect on the size of the expansion and not only to have a indirect impact because ...

Well at least from the wiki it would sound as if each biter/spitter forms his own nest/worm:

"Once they have found a suitable spot, each biter/spitter in the group dies and forms a new nest or worm. There is a delay between each member sacrificing itself for to create a worm/nest, so creating the ...

As seen in https://youtu.be/3LgELN8PVTo?t=151 an enemy expansion group of ten(10) natives only spawns two(2) spawners and two(2) worms. Totalling up to only four(4) instead of ten(10) structures, when the expected behavior would be that ten(10) new structures would be spawned.

Short:
Expansion ...