The patch from #2792765: Allow XML elements to pass through as field values to process plugins → also needs to be applied to the Simple XML parser. The referenced issue only applied it to the more advanced XML parser.
Fixed
3.0
API
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
The change introduced in this issue is causing a bug at 🐛 Migration failed with source plugin exception: Serialization of SimpleXMLElement is not allowed Needs work . It would be helpful if anyone involved in this who remembered how it works could comment over there.