What did the supplier RCA find as the root causes for the issues? Was there anything which needed to be incorporated into your processes? For example, was it from lights in the trailer itself? Do they have an inspection process which you mandate in your agreements? Were they using the vehicle for other loads? Do you restrict this in your agreements with them etc.
So if it was from a previous load for example, what your auditor might be after is an understanding with your haulier on what they transport and, if there is risk in this, how you can mitigate it. For example, can you have a written agreement with them (which may have to be a contract addendum for now) to restrict what they transport? At least for your loads? And that they inspect the lorries and keep a record of it which can be made available to you at any time etc? So it's not just understanding the risk but when the risk is known, what actions you need to put in place between you and the haulier to prevent.
OR, if it was simply that they do have a process but weren't following it, you'd need to understand with them why that was and what they've put in place to address that.
If they can't avoid transporting glass packaged items, then it's up to you if you want to put up with that risk but they may be able to mitigate it with a recorded glass breakage procedure and post glass load / pre inspection before loading.
I'd also consider pallet risk in case they are reusing pallets from a glass site.