PowerPoint automatically change to bold one of the numbering in a list that contains bold font

There is currently no direct way for doing what you're trying to do. You need a workaround:

Select M and use Ctrl+B to unbold it (This change is just temporary)
Place your cursor on the left of M and go to Insert > Symbols
Now here in the dialog box, Ensure that Font: (normal text) and form: Unicode(Hex) is selected.
Type in the character code as 034f (also known as Combining Graphene Joiner) and click insert.
Make 'M' Bold again by pressing Ctrl+B.

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