By the way, I don't think the outline behaviour is related to the \forall.

In fact, if you only type x in a formula and convert to PDF, it will still be outlined

ALSO, if you don't even create an equation, just type 1D465 and press Alt+X (which will convert to U+1D465), convert to PDF, it will still be outlined

Thank you for the effort of reproduction

Please be aware that you should use Unicode input instead of LaTeX, and you should build the equation at last.

By "selectable" I mean selectable in a PDF viewer (e.g. Chrome or Acrobat), like other text. Being able to select and copy.

This issue does not just happen in MS Word. If I use LaTeX with unicode-math package, I will get the same results (some text parts being converted to outlines when opening with Ai)
(But it's fine if I don't use unicode-math package, i.e., using the traditional Type 1 math fonts)

(The math formula in LaTeX context should be \( \forall x \in \mathbbm{R} \) ) (MS Word does not recognize \mathbbm, though)

Confirm the behavior with the file, but fail to reproduce it exactly from scratch using Word 2021.
Insert > Equation > Insert New Equation, LaTeX, pasting the code... it produces a formula, Cambria Math gets used, but R uses U+0052 instead of yours U+211d...
Still 'x' gets outline — even if I skip \forall (I suspected '\forall x' can force 'x' to get outlined...

I lack experience with PDF encoding, so I can’t see the difference in streams that force this specific character outlined :(

As far as I’m aware Adobe never encourages editing PDFs in Ai. PDF in considered to be a final / exchange format... so I don’t expect Adobe PDF Maker would ever get a way to insert Ai’s native PGF section, just to let some of us to import this into Ai to edit — something Adobe clearly discourages.

When you say 'OK for me if the text parts are just not editable in Ai, but it is NOT OK that the outlined text parts become unselectable FOREVER' — well, outlined text is not live text anymore, so it is selectable (as object) but not editable (as text). That’s what 'outlining' text does...

I ran another experiment — I tried to import the PDF into Affinity Designer. The text IS editable (something you call 'selectable'), but the 'x' is doubled (with or without the 'Favor editable text over fidelity' option enabled).
Then, if I try to copy the contents of a formula from a PDF viewed in Acrobat and paste it in Illustrator, I get the same 'doubled x'.
So I assume the positioning of the 'x' Microsoft apps use somehow forces Ai to outline it to match it.

I can’t make more experimentation without the original .docx provided (to try to export it into other formats with different settings), but I don’t think it’d matter much... this requires somebody more savvy than I to review.

The minimal reproduction PDF is attached here

The way to reproduce is posted in the previous community thread, it basically contains only a formula \forall x \in \doubleR

The PDF is generated by Adobe PDF Maker, it contains all text and graph elements within a document.
In fact, I usually draw graphs in PPT, export to PDF, and fine-tune in Ai (for example, removing some extra invisible frames or precisely clipping). The reason why I export to PDF rather than SVG is because SVG may not preserve the correct layout (e.g. an OTF bracket within a math equation)

Besides, the ReType functionality is useless in my case, because it cannot recognize the correct font. It would be better if I could manually specify what font it is.

Apart from all these, it is OK for me if the text parts are just not editable in Ai, but it is NOT OK that the outlined text parts become unselectable FOREVER. Could there be a trick that lets the unchanged outline to restore the text? Something like a pass-through technique that could copy text parts directly from the original PDF to the output?

I also notice the exception you mentioned that "a PDF generated by Ai" could "Preserve Illustrator Editing Capabilities". I think Adobe PDF Maker should have the same capability that inserts enough info to allow full edition in Ai

Just to notice — PDFs never get 'opened' in Ai, they are always 'imported'. The only exclusion of the rule is a PDF generated by Ai itself, with the 'Preserve Illustrator Editing Capabilities', which inserts the copy of the document in the native format along with the PDF copy.
For any other case — Illustrator is not a general PDF editor and never promised to keep all text in all PDFs editable when imported.

The request is clear enough though, thanks for reporting this.
If possible, share the PDF generated by MS Word, please