Thanks again for your patience. This issue was reproduced and it got filed as a defect, which will be tracked in our internal system as SOS-DEF-2655: Import/Export - Exports/Reports: Custom text box field for an item is not UTF-8 compatible for 8.25 after upgrade.
The workaround for this bug is to set the custom field text box to have rich text and assign it a template. Then navigate to the item and edit and save it.
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