This example uses large numbered images to make it easy to see how the image ordering works between the Collection in Lightroom and the Gallery Page or Client Galleries in your Format Dashboard.
New Item Position Setting
From Format Dashboard > Settings > Site > New Item Position = 'Last' (Default)
2. Add eight images (numbered) into a new Lightroom > Format Plugin Collection > Gallery Page and 'Publish'.
3. In Format > Edit Gallery Page thumbnails show order matching Lightroom's initial order.
4. Return to LR and add two new images (9 & 10) and publish
5. New image thumbnails 9 & 10 added LAST
6. From Format Dashboard > Settings > Site > New Item Position = 'First' (Changed) and Save Changes
7. Return to LR, add two new images '11 & 12' and Publish
8. New image thumbnails '12 & 11' added FIRST to Gallery.
Changing the position of images within Lightroom in the Collection will not affect the order of images when it is Published or Republished to Format, this isn't recognised, the images will either be added LAST or FIRST to the Format Gallery.