We always encourage freelancers to ensure that the escrow is funded at all times. Initially, we recommend that the deposit covers 50% of the Proposal amount.
If the project is ongoing and there are not sufficient funds in escrow, it may be appropriate for the freelancer to request further deposits throughout the project. If this is a one-off project and the deposit already covers all or most of the expected final cost then it may be that the freelancer may have made an error.
Simply message the freelancer and ask why they have made the request. If it is in error the freelancer can cancel the request.