The background turns to white
For those that experiences this, here are your options:
– Disable PNG. GD isn’t very good at converting PNG anyway, as GD cannot create lossless webp, only lossy webp
– Ask your host to upgrade GD
– Ask your host to install imagick, vips or cwebp
Reference: https://wordpress.org/support/topic/the-webp-served-instead-of-png-has-black-background-instead-of-transparent/