Instead of copying the URL from the address bar in PBase, click on "Edit Image" and then on the PBase edit page in the upper right hand corner, you will see a direct link URL sans HTML tags. Copy that and paste it into the EMBED box in AMASS and I think it will work. If it does then that answers the question of why it is behaving the way that it is.
I am trying this with my iPad to see if it works without the need to use the text editor to strip out the HTML.
OK, that worked so I now believe that I know the reason why the problem is occurring.
My other hobby is woodturning and this is a wooden bowl that I donated to a charity called The Empty Bowls Project.