Hi,
By default there is a record rule in Odoo13 to handle partners across multi companies. if you activate the debug mode and navigate to Settings -> Technical -> Security -> Record Rules, you can see a rule named res.partner company , if you didn't find it on searching it might be set as active = False, so you can click the Filter button and select the Archived and search once again.
So if it is active = False, you can set it to active=True by clicking the Boolean field Active .
In the early days of Odoo13 the rule was not there, it has be re introduced recently, see this commit and message: [FIX] base: better multi-company rules for partners
Thanks
Thank you both for your advice.
I'll look at it all and report back.