Wouldn’t it be better to try to define this as an overlapping type and keep compatibility with core types of ActivityStreams. The spec says (par 4.1):
When an implementation uses an extension type that overlaps with a core vocabulary type, the implementation MUST also specify the core vocabulary type.
So then the Create a new issue would have a: "type": ["Note", "fiv:Issue"]
How to map fiv:title and fiv:description to the content property is another matter, but doing like this would allow compatibility with the existing fediverse (if implementations there facilitate parsing the type property as an array).
And then the AP data for a more complex case would be more like in Examples using multiple Vocabularies.