Today I found that Chemical Vats from TFGM show the contaied fluid, if it is in the "output" inventory.
Input fluids when using the Vat as a normal tank are invisible / not shown
This is purely visual.

Used version of pnp: 3.2.1
Used version of TMFG: 1.3.1 CE (Communtiy Edition)
No addons to TFMG.
1.21.1 Neoforge
No relevant config (only changed to allow more than one fluid)

Reproduction is pumping any liquid into a Chemical Vat (r-click with wrench to add window).
To see if the Second (output) inventory is shown, I build a small liquid concrete production in creative, which filled the second inventory with conrete.