Default config, unlock all slots, die, respawn, you now have your startingSlots=X.
Dont matter if loseSlotsOnDeath=false or slotsLost=0.
Edit: forgot to put version of mod and MC, 1.12.2; 5.0.0
Even the dev dont test the builds?

I'm having the same issue.

Yea, I wish they would have fixed this instead of just moving to new versions. 1.12 broken