Zoo stacking + Transmutation feats

[Surrender The Throne]

Example premise:
Character has GSF: Transmutation
Character casts Bull's Strength until maximum roll (6) is reached.

Expected behaviour:
Due to the GSF, the minimum roll on each casting is 3. Therefore, it should take a maximum of two castings to reach 6.

Ingame behaviour:
Rolls after the first one don't respect spell focus feats, making it possible to roll a 2 to a cumulated 5, on a second roll.