This is working as intended.
Discourse support the 3 types:
- Full: https://meta.discourse.org/t/using-slug-only-as-url-without-id/174258
- Only ID: https://meta.discourse.org/t/174258
- Only Slug: https://meta.discourse.org/t/using-slug-only-as-url-without-id
But if the slug starts with a number we special case and look for the first number as an ID: