I'm not sure where I may have missed it. But I noticed the more powerful dragons have saves as fighter higher than level 14 how are those rolled?
The saving throw table for fighters says 14+. So I would just use that row.
You could also use the description of the Fighter in the core book, which says that they advance in saving throws by two points every three levels of experience.
I was thinking that but since it was specified I wasn't sure if something different was intended.
I might give this a try.
I’ve always assumed that the saves keep progressing.