Title in theorems and axioms

[dbrogioli]

What version of myst-parser are you using?

myst --version v1.8.1

What version dependencies are you using?

node --version v24.13.1
npm --version 11.8.0

What operating system are you using?

Linux

Describe the Bug

The title of theorems and axioms does not appear correctly. The attached image is from the [MyST guide itself]
(https://mystmd.org/guide/proofs-and-theorems).

The image shows the MyST code, where the axiom title is "Completeness of $\mathbb{R}$", and the output, where the title only contains $\mathbb{R}$. In general, the text does not appear completely as title of the theorem.

Expected Behavior

I think that the expected behaviour is the the text after ":::{prf:axiom}" should be rendered as the title of the theorem.

To Reproduce

The example reported in the figure, appearing in the guide itself, shows the problem:

`
:::{prf:axiom} Completeness of $\mathbb{R}$
🏷️ my-axiom

Every Cauchy sequence on the real line is convergent.
:::
`

The resulting title is just "$\mathbb{R}$" and not the complete title "Completeness of $\mathbb{R}$"