Is there a single tax fraud case that does not involve some sort of other violation? Probably not. From the link I posted, mortgage (loan) fraud gets 20 months on average. For failure to report a foreign bank account I have seen cases with up to a year in jail. The amounts still don’t add up to 19 years. Manafort was certainly not a blameless cherub, but his prosecution and recommended sentence is clearly politically motivated.
So where did 19 months come from? It was from sentencing guidelines.