Photoshop flattening transparency when saving to PDF

I'm confused because I thought I used to be able to do this with Photoshop, but I must be doing something wrong...

I have an image with white text over a transparent background. You can sort of see here...

When I save to PDF, the white text gets flattened into a white background as if I'm saving a jpg.

I've tried saving with the following settings:

Upon opening it in Adobe Acrobat, I don't see any evidence of transparency:

Is this normal? I can't recall how transparency should show up in Acrobat, but the printer also said that the transparency was lost when I sent them my file.

I tried to open it in Illustrator, and got this dialog box:

But once it opened, I could see that it was showing up if I dragged the art to the artboard. 

I'm using PS 24.4 version on a 2019 macbook.

I've tried doing this with the latest version of "save as" as well as the "enable legacy save as" option you can set in the preferences. Both seem to produce the same effect. When I sent this pdf file to the printer, they told me that they couldn't access the white text and that it had been flattened into the background. I'm confused...