Allocation Schedule changes expense class

We have a simple allocation schedule that is used to allocate facilities expenses by department. The criterion are simply:

Any transaction with a dept of 'facilities' and a location of X
(X is the office location)

There is a separate schedule per office, and the destination is setup as headcount percentage weights per department (ie 25% sales, 15% F&A, 12% development etc)

The problem is that when we put a class on the original expense (typically a vendor bill), the allocation entry wipes the class value out when run (the bill has the class, but the individual allocations do not. I see that we can set a specific destination class on the allocation schedule, but we cannot set 'use source class' in the same way that we can set 'use source account'.