So, this is what's happening:
Before CC2025, if you open a file that uses Type 1 fonts (which AG Old Face is), they would show as missing in the manner you have known so far.
CC2025 does things a bit differently. When you open an .AI file, if the fonts are either missing from the System, or Type 1, it will pull the outlines from the PDF portion of the .AI file (as they are embedded in the file after all), much like how it works when viewing a PDF in Acrobat. This is why it says "PREVIEW". You can make a PDF of the file at this point and these will carry over. What you CAN'T do (just as in Acrobat), is edit the files in any way unless those fonts are available in the System. If you attempt to do so, it will warn you and replace them with the default font. And since they are Type 1, you wouldn't be able to anyway.
This is also why if you place an .AI file in InDesign, it works as it's using the embedded outlines in the .AI file.
What you may want to do is uncheck the box that says "Don't Show" so when you open the file it will bring this to your attention. But now that you know this, you can watch for it going forward.
Also: not sure why you didn't just get your client to send you another PDF, nor why you didn't have the client approve a proof.
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