This looks like Illustrator 2026 is reading the embedded Illustrator data, while older versions fall back to the PDF representation. One useful check is to open the file in Acrobat and look at the Layers panel. Are the original layers independently visible and switchable there? If they are, the PDF still contains recoverable layer groups and there may be a way to bring them into Illustrator without rebuilding everything manually.
This looks like Illustrator 2026 is reading the embedded Illustrator data, while older versions fall back to the PDF representation. One useful check is to open the file in Acrobat and look at the Layers panel. Are the original layers independently visible and switchable there? If they are, the PDF still contains recoverable layer groups and there may be a way to bring them into Illustrator without rebuilding everything manually.