Split File Thumbnail

Hello,

I noticed that my .tif files thumbnail are split, as if showing two layers. However, that does not happen to downloaded files. My files are 16-bit stretched astrofotography files and flattened in the end. The problem is that when processing in  third party software, that software acts as if the image was indeed split.

That split in the thumbnail happens after processing - the original file thumbnail is just one image.

Attached you can see:

Letft top - split thumbnail

Left bottom - downloaded file

Right - processing output

Does anyone have a clue of what is happening?

Thanks in advance.