double strike stacking: zeal spell and perfect twf?

[redoubt]

I'm playing a pure pali vanguard. I've been taking everything I can to max out double strike for damage.

At 27 I took perfect two weapon fighting. I appears that after taking the feat my double strike went from 42 to 47. This is expected.

Now, when I cast zeal my doublestrike only goes to 52 instead of 57.

The spell is a sacred bonus (I don't think anything else is.)

I do not know the bonus type of the perfect TWF feat (but would assume type: FEAT, which should stack with everything.)

Any ideas what is not working here?

Thanks.

[Infiltraitor]

Tested on Lammania, unable to replicate bug.

[Eth]

I guess you suspected PTWF because it's the only 5% source? What are your other doublestrike sources?
There aren't any other sacred doublestrike that I know of. Celestial champion used to be sacred initially but was changed later.