"I'm finding it hard to understand why no attempt was made to continue with the the offending flight-plan handled by the manual system."
Possibly that the errant flight plan could not be identified - if it could then why didn't the software throw it back.
Some very odd numbers / usage in the public domain.
"1 in 15m flight plans over five years" could be 1 in 3m per year; either value is meaningless because they would be forecasts - technical guess, objectives - which were not met.
interesting views in https://snafucatchers.github.io/ #111