Hi,
I tried to use Plugins / Images / Print Pictures to print multiple selected images.

After changing scaling settings in the Print Dialog the new settings get only applied to the first of the selected fotos.
For example:
I changed the scaling settings from "Adjust images to page size" to "15.00 x 13.00 cm"
Then I clicked on the button Use as Default.
The result is: Only the first image is resized correctly. The others remain in full size.

My Gwenview is in Version 18.12.3 under openSUSE Leap 15.1.

I was able to find a workaround.
After reopening the print dialog "Plugins / Images / Print Images" the previously set default value will be applied to all selected fotos.

Well, I am dealing with the same issue right now. I have even tried the "workaround," reopening the print dialog "Plugins / Images / Print Images but it also does not work! I had to print some fantastic images for an event that is going to take place the next week, and this damn printer is making me sick! I have found some really outstanding pictures, and I am the host of the event. I have to make it all perfect. You can look here and see all the pictures that I have selected for this occasion.