Yeah, SELECT FOR UPDATE should overwrite the broken xmax value and thereby fix it, I expect. However, I don't see anything in the release notes suggesting that we've fixed any related bugs since 9.6.10, so if this just appeared then we've still got a problem :-(. Did anything interesting happen since your last successful autovacuum on that table? Database crashes, WAL-related parameter changes, that sort of thing?
The last autovacuum of that table was on Dec 8th, the last auto analyze was Dec 26. There have been no schema changes on that particular table, database crashes or WAL-related parameter changes since then. We've done other schema changes during that time, but otherwise the database has been stable.