When working with Figures in a pdf, you sometimes need to add Bounding Box Parameters.
You do not have to place the original figure in the background. Because when you do, the figure tag does not remain in its proper location in the tag tree.
Here is a shortcut
Save the alt text to your favorite editor program, such as Notepad.
Turn on the Touch Up Reading Order (TURO) tool.
Highlight the figure with the mouse by drawing around the figure.
Select figure on the TURO tool.
Right-click the tag in the tag tree and add the Alt text.