When I export a bunch of images to Flickr by using the built-in share to flickr, I see the images getting exported but they all appear as max 1024 pixels on either side. I did select "Actual size" when exporting so it should use the full resolution. And I think it did work fine in a previous export.
Just wait a little longer. What happens is that it starts by uploading 1024-sized images, and then later replaces them with high-resolution versione. So if you check flickr while it is still working you will see the lower-resolution versions but wait and reload and it will be high-resolution.