[1.2.304] Strange items...

We can't possibly know at this point. It's safer to think that it's +25% damage for now, since +125% would be the most broken thing possible. Of course the chance that these skills are PQ based or have some sort of limitation is extremely high anyway, so it probably doesn't matter.

Are you implying that the stun mastery change is a glitch?

There is no reason to suspect that Stun Mastery is intended to add 300% damage with SE.

Similarly there is no reason to suspect that self SE is intended to add 125%, but it could all the same.