When you post usage on a job, G/L entries are posted based on the items inventory posting group, and the items product posting group combined with NO business posting group:
From the general posting setup, the inventory adjustment account is taken from the combination without business posting group.

This means that a job usage posting is treated the same as an 'ordinary' inventory adjustment: as a positive or negative correction from an item journal.
However this is financially not the same, and should not be the treated the same.

Right now, the only way to treat the two postings different is by selecting a business posting group in the item journal, but that is a manual action and thus sensitive to errors.
It should be better to change the job usage posting, and post to G/L with the customer's business posting group.