Only able to convert math regions one by one

I am working with a school that hopes to evolve from using MathType to OMML for the documents they provide to their students. They have provided me with this sample sheet so I can provide a step-by-step guide.
[Exercise 8B N24PB.docx](https://github.com/user-attachments/files/24085486/Exercise.8B.N24PB.docx)

I can convert individual expressions by following these steps:

- Select the expression
- Press Shift+F10 or right-click
- Choose Convert to Office Math

This works well—the math looks great and reads beautifully (especially in UK English, as I always remind Neil!). The formatting still needs adjustment because the worksheets are designed for all students, including those with low vision, so the content must be large and bold. But overall, the conversion process works.
**The problem:** Doing this for each expression manually isn’t practical. The Convert All option appeared once but I can’t get it back.
I tried searching for Convert math regions using the Microsoft search bar (Alt+Q), but that doesn’t work.
I’ve successfully converted several other documents, so I’m wondering:

- Is this issue specific to this document?
- Could it be related to my machine? (I’ve tested on two different PCs.)
- Has Word’s behavior changed?
- Is this feature buggy?