PNG files (not set to “True Color” palette) aren’t rendering properly

I inconsistently found that PNGs weren’t properly converting to WebP format. I saw a PHP warning that explained the issue when I tried to regenerate them with CLI.

PHP Warning:  imagewebp(): Palette image not supported by webp in /var/www/html/domain.com/wp-includes/class-wp-image-editor.php on line 585
Warning: imagewebp(): Palette image not supported by webp in /var/www/html/domain.com/wp-includes/class-wp-image-editor.php on line 585

So, of course I took these errors to AI… and here’s the response it gave:

This warning occurs because the imagewebp() function in PHP’s GD library cannot convert palette-based (indexed color) images—common in some PNG and GIF files—directly to WebP. WordPress’s image editor hits this limitation when handling uploads or regenerating thumbnails.

Why This Happens
• WebP conversion with GD requires images to be “true color”; palette images (indexed color) aren’t supported by imagewebp(), resulting in the error.
• Many modern PNG compression or optimization tools convert images to palette-based PNGs by default to reduce file size.

How to Fix
Convert the image to true color before passing to imagewebp().
• For PNGs and GIFs, always check the color mode and convert to true color before calling imagewebp().