PDF prints flat unless opened in Photoshop — how to tag DeviceRGB?

I’m on Windows 10, running two BenQ SW321C monitors (both calibrated) and an Epson ET-8550. I’ve run into a color issue with PDFs that were created on a Mac.

• If I open the PDF in Acrobat or Foxit and print directly, the print looks flat/washed out.

• On screen, if I switch my monitor into sRGB mode, it matches that “flat” look.

• If I pull an image out of the PDF and into Photoshop, Photoshop warns me there’s no embedded RGB profile. If I assign AdobeRGB (my working space), the print looks right.

• If I export the whole PDF as a JPEG with AdobeRGB embedded, that prints correctly too.

So my guess is the PDF images are DeviceRGB with no ICC profile. Acrobat/Foxit fall back to sRGB, while Photoshop lets me assign AdobeRGB and get a good print.

Here’s what I’ve tried in Acrobat:

• In Preflight, Convert Colors seems to work. Converting to AdobeRGB goes through a process and the colors on screen look identical. Converting to sRGB and they look flat. But the “embed on output” option is greyed out, and the prints are still flat.

• I created a custom fixup to assign AdobeRGB to DeviceRGB objects, but the new PDF still prints flat.

• I tried converting to PDF/X-4, but I can’t find a way to actually tag those untagged objects with AdobeRGB.

So my question is: what’s the right way in Acrobat to force AdobeRGB onto untagged RGB objects, so the PDF prints the same as it does out of Photoshop? Appreciate any help. I’d like to land on a reliable workflow so I’m not exporting these PDFs to Photoshop just to print them.