Currently, /api/issues/bulk_change only sets a comment on issues that are either affected by a change of type, or a change of severity. It's not possible to set a comment, without changing the type or severity. This is not reflected in the documentation, and is misleading
The following looks valid, as per the Web API documentation, but it returns an error:
Error: "At least one action must be provided"
This is misleading. We would expect the comment to be set on both issues, but only issue-bug will get a comment:
Quick win is to make the documentation more explicit. Note that there's a feature request to support only passing comment, making it a valid action.