So, similar to the response from @mikelyndersOKCC in the Automating Fulfillment Workbench Via Epicor Function - #4 by adaniell link you reference, I just needed a bit of patience for the previously deferred entries to make their way out. Calls to my function now result in the on hand quantity immediately reducing.
This potentially explains other instances with similarly odd behavior. I’m sure this isn’t a panacea, but hopefully another arrow in the quiver! Thanks for your help! Epiusers is truly one of the best things about Epicor 