Version 0.1.19 on Windows.
There seems to be issues with the interaction of spell gem slots with the Druid Sovereignty perk and the Bloody Ossein. With maxed Companion on Druid and with three creatures, including the Bloody Ossein, in my party and without the Sovereignty perk, I have 18 spell gem slots (it does take into consideration the Companion perk 5 x 3 = 15 additional spell gem slots).
If you then activate the Sovereignty perk with the Bloody Ossein in your party, you lose 3 spell gem slots (15 spell gem slots total). Or as I just tested when duplicating this issue on a copied save, I now have 23 spell gems slots, though you can only use 18 of them.