quantity variation first quantity input is calculated twice

Hi,

we set up a meta group with quantity variation and price matrix for selling t-shirts.
If we raise the quantity of a shirt-size to 1, the total price is twice. If we raise to 2, the price is correct. This issue works for every first input of a quantity (if i insert 10 as quantity, i get the total of 20 pieces).

for example: 1 shirt cost 15,90 EUR
If i insert a quantity of 1 for a shirt with size L, the total is 31,80 EUR. If i change quantity or insert an additional quantity, the price is calculating correct.

woocommerce 3.7 | ppom 18.4 | ppom pro 15.6

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