bitcoin sent from coinbase will come from a wallet they control. if you sent bitcoin or bitcoin cash back to it they now control those funds.
the proper course of action is to document the process, what addresses was used, which transaction ids was assigned and what blocks did the transactions get mined in, then contact coinbase customer support and request that they help forward those funds to an address you control.
do it as soon as you can, but be aware that you might have to wait until coindesk supporyscoinbase supports BCH to withdraw them.