The XFDF export nested one <field> element per period-separated part of a field's full name. A /T can hold periods of its own, so a name can have far more parts than the field tree has levels. Past 64 levels the import refuses the file the export wrote, and around 100,000 parts the recursive write overflows the main thread's stack and aborts the process.
The export now nests at most as many levels as the import reads back. The parts past the last level stay in the deepest element's name, and the import joins them back into the same full name.
Unhumanje
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