Is the issue we get from the post function v2 or v3?

The issue format within the contents of the HTTP POST our connect app gets when a post function is triggered is/was typically that of the v2 issue, i.e. the description is a string.
This is also still true for my (developer) Jira instance. But on the (sandbox) Jira instance of a customer the error message when accessing the description (class java.util.LinkedHashMap cannot be cast to class java.lang.String) suggest that he gets a v3 issue.

So the question is, is the format v2 on some instances and v3 on others and is it going to change to v3 on all instances?

The documentation does not specify this: