After some further investigation, it appears that a mod (Secretas & Frozeta) adds a compatibility recipe with the Cubium planet mod, which requires 2 fluids and produces the planet's science pack. (pictured below)

However, the assembling machine, when set to processing units, shows 2 fluid input icons in alt mode. the one with the pipe sticking out of the assembler works as normal, however the one without the pipe doesn't, which leads to some minor issues (i have already rotated my buildings incorrectly once before because of this)
a picture of a small blue circuit line in alt mode below:

This also appears with the "easy electronic components" recipe, which takes water as its only fluid input. I have not noticed this with every fluid recipe in an assembler however, only these two.


