When I use /dev/null and an active QuantumFlux Item Magnet, /dev/null does not take the configured itemstacks - without magnet, it works as expected. I do not know this is your side or OpenBlocks side, but I kindly ask to investigate this issue. Also, /dev/null works with many other magnets correctly.

Affected versions:

QF: 1.2.3
OB: 1.4.3
ModPack: FTB Horizons II with disabled Redstonic mod.

Crossref: joekaiser/QuantumFlux#65