A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
Based on my research, mathematical bold appears to be treated as a semantic mathematical style rather than ordinary text formatting. The OfficeMath documentation also indicates that mathematical styles can represent distinct mathematical entities within math zones, rather than simply changing visual appearance. Therefore, it is possible that the OMML-to-MathML conversion process applies styling at the individual character or token level. However, I could not find any Microsoft documentation that specifically describes how bold mathematical numbers are expected to be represented during export.
Reference: OfficeMath
To help investigate this further, you may also consider posting in the GitHub Community Forum, where you can engage in more in-depth technical discussions with developers and accessibility specialists. Additional input from individuals with experience working directly with OMML, MathML, and accessibility tooling may help provide further insight into the behavior you are observing.
Please feel free to correct me if I misunderstood your request. If you have any additional concerns, feel free to comment below. I would be more than happy to assist.