I have been exported via XML perfectly well, but I added some custom
fields and now the when the .xml is imported I get multiple rows per
bug.

I can still export and import via XML into the old style spreadsheet
before the changes and get one row.

Even if I just import the bug-id and one other element I get 2 rows
per bug.

Is an old schema hanging around? Is there some limit on the number of
fields exportable via XML?