Hi M,
Thanks for bringing this to our attention. For now, you can fix this by using an automatic width in CSS. To add custom CSS, you can go to Customizer > Custom > CSS in your dashboard.
.woocommerce .quantity input[type="number"], .woocommerce-page .quantity input[type="number"] {
width:auto;
}
You could also just give a specific width that’s a bit wider like this:
.woocommerce .quantity input[type="number"], .woocommerce-page .quantity input[type="number"] {
width:4em;
}