mdinclude not using correct header names if numbers are used

[CorrieBartonVualto]

Hi, I've noticed an issue with ids for headers when using .. mdinclude:: file.md if the header contains a number.

For example, within file.md I have the header: #### March 2020. I'd expect the id for this to be march-2020, however, when displaying this file within a .rst using mdinclude, the id becomes id1 if a number is used.

#### March Twenty Twenty has the id march-twenty-twenty like you'd expect, but I'd prefer to use numbers.

Is this a known issue or am I doing something wrong? Thanks