Hello,
This issue is happening because of this CSS you added in Theme Settings:
.wd-action-btn .wd-action-icon::before {
color: #fff !important;
}
This forces the wishlist icon to stay white everywhere, so on the single product page (white background), it becomes invisible.
You should remove this global rule and instead apply a more specific style only where needed, or override it for the single product page, for example:
.single-product .wd-wishlist-btn.wd-action-btn .wd-action-icon::before {
color: #d62128 !important;
}
Best Regards,