incorrect tax in PO

Hi Experts,

We have an issue in which PO item tax is behaving inappropriately.

1. When we click on condition tab in PO line item, it shows tax.

2. After step 1, When we click on the invoice tab and check the taxes button, the tax becomes 0.

3. After step 2, when we go back to conditions tab in PO line item, the tax becomes 0.

This is really strange. Can anyone help me on this ?

Thanks and regards

Meenakshi