I have a demo project whose fields I changed to reflect a new workflow. I changed just the fields in this one project via the "Edit Project" screen in the "Fields" tab.
For example I copied "type" and "state" to just change in this project.
Now ONLY when I query "/rest/issue/byproject/EXAMPLE?filter=some+filter", so byproject with a filter, then I get an empty result. When using no filter then I get all issues for the project.
Every other project works. It's only this demo project that can't be queried for its issues, but ONLY when using byproject WITH a filter, "/rest/issue/byproject/EXAMPLE?filter=some+filter".
So I guess I screwed up with copying the fields somehow?