This is actually still in the original Bioware form, meaning we didn't change the script at all.
What's going on here is that the "levels with the caster" is rather granular, not continuous.
From level 1-7, you get the Shadow you've been getting
From level 8-10, you get a Shadow Mastif
From level 11-14, you get a Shadow Fiend
Above level 14, you get a Shadow Lord
So my guess is it's working as intended. If you don't see any changes at level 8, let us know.