According to the OpenAPI spec, we're supposed to ignore any additional properties defined on a reference—which would include default unless I'm missing something.

The "right" way to do this might be with allOf?

Sorry for not getting back to you earlier. I needed a bit of time to deeper understand the reference you made, since it seemed quite straightforward (but contradicts what I wanted it to say :))

However (and I'm still not entirely sure I've wrapped my head around this correctly) there is a distinction made between "Reference objects" and "Schema objects that contain a $ref keyword".
After some searching, I found this: https://learn.openapis.org/referencing/overview.html#a-taxonomy-of-references.

Isn't this an example of the 4th row in the table, whereas a direct reading of the spec would be an example of the 2nd row in the table?
The remark that 'this is more-or-less equivalent to using a one-element "allOf"' seems to agree with your suggestion to use allOf.

Another interesting fact is that this schema structure is generated by pydantic (from at least 2.8 up to 2.11.3 which is the latest version at this time of writing). So it seems that it's either a bug in how pydantic generates the schema, or how we interpret it :)
What do you think?