In Sitecore Commerce Connect, Manufacturer and ProductType fields are based on deprecated "Treelist with Search" field type which, in turn, causes rendering problems while working with items based on the "Product" template. This field type is deprecated beginning from Sitecore CMS 7.2: https://sdn.sitecore.net/Products/Sitecore%20V5/Sitecore%20CMS%207/ReleaseNotes/ChangeLog.aspx.
To resolve the issue, upgrade Sitecore XC instance to Sitecore XC 10.0.0 or later.