Dexterity Modifier & Spell Stacking

[Rhangalas]

I'm noticing some wierdness with the dexterity modifier in relation to these spells:

Dragonskin
Stoneskin
Cat's Grace
Bull's Strength
Bear's Endurance

When I have just Dragonskin cast on me, the maximum dexterity modifier is 3, as usual.

Usually, when I stack Stoneskin afterwards, the modifier drops to 1.

Then I cast the 3 attribute buffs next: Cat, Bear, then Bull. The modifier drops to (none). When the buffs expire, the modifier goes back to 3 even though Stoneskin is still in effect.