Filleting Edges Tangent to the Edges to be Kept
You can fillet the edges which are tangent to the edges to be kept by following selecting edges in correct order.
You select one edge you want to fillet (the edge shown in red with a radius value), then two edges you do not want to fillet (in purple), keeping in mind that the selection of "keep edges" propagates the selection of the edge you want to fillet. The preview shows that the fillet operation will apply onto two additional edges that are shown in red dotted lines. For more information about the Keep Edge option, see Creating Edge Fillets in the Part Design User's Guide.
Result
Only the edge you explicitly selected is filleted.
In this particular case, the edge to be filleted and the edge to keep
become tangent: the fillet does not cross the edge to keep and stops.
Workaround
Create two different fillet features: