Disclosure: I am a newbie to text mode.
I wonder if it has to do with "resolve project entry date" not being a field that exists on the Issue object level per the API explorer. For Projects, "converted issue entry date" does exist, so from that perspective, it makes sense that this could work for your project report and not your issue report.
If we can only calculate date differences on the same object or on the parent of an object, this may not be possible for an issue report.
If we try thinking through this another way: For this use case, what purpose would the issue version of the report have that wasn't already covered by the project version of the report? Is it a visibility thing where you are already looking at a list of issues anyway so having that extra column of data would be easier than having a totally separate report? (If so, I feel ya...hate having to look multiple places for info.)
Is there another potential flag that could be used in place of resolve project entry date? For example in my environment, when an issue is converted to a project and the project goes into Current status, it means the corresponding issue goes into In Progress. Could you capture that timestamp (via a calculated field that you would need to create first and attach to the issue) that the issue goes from New to In Progress status change and calculate from that date?