Thank you for your post!
I was able to reproduce this issue with your specified steps:
1. Created an XML DDR from an existing database file
2. Changed .xml to .txt
3. Imported into FileMaker as a new Database File
*I'm only having 7 fields created, as you see.
I am forwarding this information to Testing and Development for further feedback. I will respond back here when information is available.
Our Development and Testing teams are already aware of this issue. It appears the issue manifests after the 501st record.
Your issue has been attached to the original report. When more information becomes available, I will post again.