Maybe GSpot is wrong?
Use MediaInfo, much more reliable.
You can set the BFF flag and still have a progressive footage. Progressive overrides the field order. In other words for an interlaced source the field order is mandatory, for a progressive it is irrelevant.
To close the loop, could you run again the AVStoDVD project using default settings (without touching anything in Title Edit)? Then, please post the complete AVStoDVD project log file.
Bye