Problem with extension when I use multiple cores converting process

[IzoFox]

Hi!
I have a bug.
When I convert CR2 images, or other format, like WebP, or PNG to JPG and I enable checkbox "Use multiple CPU cores" and set them to maximum (32 threads in my case). I get converted images, but they are with extension of original images.

Like they were CR2 and after converting to JPG they have CR2 extension.
If I will convert them twice and more, the extensions will be changed to JPG.
If I will convert them in one thread, the extensions also will be change to JPG correctly.

I've uploaded a video with the problem.
https://drive.google.com/drive/folders/ ... sp=sharing

Thank you!
I will wait for your answer.

[xnview]

do you have enabled 'Preserve extension'?

[IzoFox]

do you have enabled 'Preserve extension'?

Yes, it have been enabled.
I've disabled it and it works good now, I think.

Why this checkbox don't have affect, when I convert in one thread?