AP Prepayment

We want to apply the prepayment to the invoice that we received from a vendor. The invoice was matched to the purchase order and since there is a tax on the invoice provided by the vendor, a manual tax line was entered. While applying the 2 prepayments against this invoice, 2 more tax lines got applied to the invoice. Why are the tax lines being added.

Prepayment Details:

0038252IN	19,680.50
	1,746.64	Tax
0038847IN	11,808.30
	1,047.99	Tax

Invoice Details:

Line	7,872.20
Tax (Manually Added)	698.66
Total	8,570.86

Now when the prepayment is applied:

It's also adding additional credit for the tax which we don't want.

Why is the process adding the additional lines - (196.80) and (118.08)