Thumbnail in culling layout not working with 2 images


following an advice from @s7habo in one of his videos, I am trying to compare an edited image with its JPEG. It seems that the functionality does not work when the selected number of images in culling layout is 2:

When selecting more than 2 images in culling layout it seems to work:

I was reading the darktable manual and some interesting posts about thumbnails but I cannot say if it’s a bug or a feature. Has someone an idea?

I’m using version 3.9.0+1744~gebe766dba current OSX Build - #505 by MStraeten