I did some digging.

It looks like what is happening is:

1. A yaml code block is identified and processed https://github.com/symfony-tools/docs-builder/blob/main/src/Directive/CodeBlockDirective.php
2. This class is then responsible for highltighting https://github.com/symfony-tools/docs-builder/blob/main/src/Renderers/CodeNodeRenderer.php
3. For yaml it internally uses this project https://github.com/scrivo/highlight.php

Possible fixes:

1. Override yaml highlighting: https://github.com/symfony-tools/docs-builder/blob/main/src/Templates/highlight.php/README.md, most probably this line: https://github.com/scrivo/highlight.php/blob/d1c6b0956a2b0d3efc137e4d8c5bf5c5e220bac1/src/Highlight/languages/yaml.json#L84
2. Use another (maintained) library such as https://github.com/tempestphp/highlight, it does not seem to correctly highlight yaml numbers though https://github.com/tempestphp/highlight/tree/main/src/Languages/Yaml/Patterns

I think the first option should be the most straightforward and the less risky. The second option may be better in the long term.

On the symfony part, it looks like https://github.com/symfony/yaml/blob/fa34c77015aa6720469db7003567b9f772492bf2/Inline.php#L695-L738 is responsible to convert numbers. From my understanding we can have _ anywhere: _32 will be considered as a number (TODO: test).