Reproducing steps:
Launch CRM
Click on Marketing
Click on Activity
Select Campaign Response from the list of Activities
Launch any Record
Behavior:
1:- If a value is specified in the Customer field, the Company Name, First Name, Last Name, Phone Number and Email fields will be blank and in a locked state.
2:- However, if we specify values in the Company Name, First Name, Last Name, Phone Number and Email fields and save the record, and after that if we try populating the Customer field the following behavior is observed:
- When we try to specify a value in the Customer field it will remove the existing data from the Company Name, First Name, Last Name, Phone Number and Email fields and lock the fields. Now if we remove the value from the Customer field the other fields will be unlocked for editing, but the existing data from these fields are removed.
- The only workaround to avoid this loss of data is to turn off Auto Save feature. This is not a viable option for the customer.
- The data which is lost can not be gained back
Ask:
If we update or select Customer field by mistake and we don't want to make changes to the record, we should be notified before selecting a value in the customer-that our data will be lost.
Or
Once we remove the value from the Customer field which we selected by mistake--Our existing data should still be visible on the page.

Dynamics 365 Sales (administrator)
Thank you for your feedback. We are declining/closing this idea as it has been open for more than 2 years and has not received sufficient numbers of votes. If the idea is still valid, request you to re-submit the idea.