Hi Andrew,
I understand you would like to display a customer column in the search result page in SharePoint Online. Please correct me if I misunderstood it.
After doing much research, I found it’s not possible to do this in SharePoint Online.
Based on my research, showing customer column in the result page needs to add the customer column as managed property in SharePoint Admin Center. However, SharePoint Online doesn’t support editing the managed properties for the time being.
In your situation, I’d like you can provide the guidance you followed here, so that I can check if the guidance is sensible, as well as try to reproduce the problem you encountered on my side.
If my above information doesn’t help you, please let me know.
Thanks,
Evan Zhang