Premiere Thumbnail Problem - First frame is not showing as a video thumbnail - Premiere Pro Issue!

I have a wired issue, and this is clearly Premiere Pro bug. 

I have just finished the chat with Adobe support and they asked me to post this issue here. 

So, I would like to create a simple video. A few pictures showing one after another and as a first frame I set a image that I want to show as a thumbnail when the video is exported. So when you look at the export folder you see the first frame showing as a video thumbnail. 

But this is not happening, my video thumbnail is a third image from the sequence not the first one. 

I understant I can add a thumbnail uploading to the YouTube but this is not what I want. 

I would like to add this video to Facebook and Instagram post there thumbnails are not allowed. 

So, the video must have thumbnail, which is basically the first image in the sequence. 

Now, wird stuff...

When the first picture is set to 1s it won't show as a thumbnail in the exported video, but when I make the lenght to abour 3s, the first image is showing as a thumbnail, it might be hard to understand to I made a video showing clearly this issue. 

Why I bother about this thumbnail? Because Facebook not allow to add the thumbnail (when you post together to FB & IS) so this must be done at Premiere level, but I don't want this first picture to be 3s long on the sequence in fact I will prefer to keep it under 0.5s but this is not possible because when the picture lenght is lower than 3s this picture is ignored. 

I never expected that making so simple video is so hard. 

Hope to hear from Adobe soon...

Thanks 

PS. 

Sorry for the speed I've tried to match 47MB Max upload file.