basically this is a pnuematicraft fluid tank but some other modded tanks work as well. The combustion generator/dc motor will fill its internal buffer without actually consuming any liquid from the tanks.

https://user-images.githubusercontent.com/26591/284781929-1565922b-e826-4ec9-9a4c-32d462d3ed8e.png

Originaly reported here:
FTBTeam/FTB-Modpack-Issues#4306

This has been fixed and will be out with the next update

well the lubricant one where it's happening is completely stock electrodynamics json recipe but the lpg fuel is this:

lpg.json

send me the json for the fuel type and I'll see if I can replicate it in the new update we're working on.

happens with crude oil too

The update with the fix has been released