It seems this plug-in (v2.6) doesn't work well with the zc group pricing (zc 1.57d). The tax rate is calculated based on the sub-total rate (before the group discount). The discount amount by group pricing is not used to calculate the tax. In the settings of my group discount, I set "Re-calculate Tax" option as standard, so the tax should be based on the sub-total minus discount amount.

I tried to set this option as "none" or "credit note", the result is unchanged. So it looks like no matter which option I set, it will be treated as "none".

This bug is not critical but hopefully it can be fixed later.