When does ImageOutputType.webpThenJpg fail to convert to WebP and fall back to JPG?

[Yoojkim]

I've tried converting various JPG and PNG images, but all of them are being converted to JPG instead of WebP. I confirmed this by reading the first few bytes of the converted files, which indicate they are in JPG format.

Could you clarify under what circumstances ImageOutputType.webpThenJpg fails to convert images to WebP and falls back to JPG? Are there specific conditions or limitations that cause this behavior?

Thank you in advance for your help!