ZCv1.56c
OPC v2.3.2

It doesn't feel like an OPC related issue.

Checking out, as a guest, I enter

Street Address
Address Line 2 [left blank]
City
County/State/Province [left blank]
Post/Zip Code
Country

In PayPal EC Checkout my selected address is

Street Address
City
County/State/Province
Post/Zip Code

On completing the order with PayPal and returning to Site I get this message:

The delivery address that you entered (Name, StreetAddress, City, , PostCode, Country) was replaced by the address you selected at PayPal. Please review your order and contact us if an update is needed.

Reviewing the order on the checkout_success page, the delivery address is:

Name
StreetAddress
City, City
PostCode
Country

The blank State field in the checkout process has been populated with a duplicate of City.