After further investigation, with Roy’s assistance, this has been fixed.
It is an issue that will only appear under specific circumstances, such as a restore of the database at a certain point in the upgrade sequences.
Should you experience an identical error message to the one Roy attached at the start of this thread, the attached replacement for the file kiwitrees\includes\db_schema\db_schema_29_30.php will fix it.
If you don’t see that error message, there is no point adding this fix. It will be included in the next upgrade.