If the Acrobat Checker tool or Validation software indicates a Figure does not have Alt Text, yet the Figure tag does have Alt Text, the problem may lie in how the PDF file was created.
Image Duplication - a possibility
If the file was created with an older software program or run through the Adobe Distiller program, images may have been duplicated.
Check for this by manually associating the image to a Figure tag in the Tag Tree.
If two or more image containers appear,
activate the Edit tool,
hover the mouse over the image until the cursor becomes a 4-sided arrow, left-click and pull the image. This should display the duplicate(s).
Delete the extra image(s).
In the Tag Tree, the image containers should disappear. You may need to artifact any blank containers.
With Tag Annotations activated, use the Cut option to temporality remove the remaining image.
In the Tag Tree, the remaining image container should disappear. If it does not or if there is a blank container, artifact and also delete the Figure tag.
Paste the image. It should remain the same size and in the same position in the file.
In the Tag Tree, create a new Figure tag.
Close the Edit tool and activate the Accessibility tool.
Activate the Touch Up Reading Order tool and move out of the way.
Select the Figure tag in the Tag Tree.
Use the mouse to draw around the image, right-click the Figure tag and select, Create Tag from Selection.
Provide Alt Text on the Figure tag.
Run the Checker or Validation software again.