I'm not sure to which property you're copying this? a custom property? a default one?
I'm wondering if maybe you have workflows errors and/or automatic properties change that cause that. You can see the source of the change if you click on details when you hover your mouse on the property value in the record.
In any case, I would recommend connecting with HubSpot Technical Support, as Support is included in your subscription and they will be able to provide real-time assistance for this matter, where you are able to share screenshots and specific examples about this.