propellant mass lbf vs lbm

[cwarlock]

The propellant mass is outputting in lb and all of my other units are set to imperial as well. This output is lb force not lb mass. When the unit for propellant mass is set as kg the unit conversion confirms that the original output was in lb force rather than lb mass.

I'm super new to the software so I may just be misunderstanding something somewhere?
Thanks :)