What does diplomatic issue mean?

In what context ?

In general... like an example of a diplomatic issue... pertaining to the department of state or the national security council

You could be speaking of an issue meaning a subject, or an issue meaning something issued.

Can you use the term "diplomatic issue" in a sentence?
... are any of these links speaking of "issues" as you are thinking of them?

A diplomatic issue would be one that referenced diplomacy. Diplomacy is the art of negotiation between countries over disputes.

It would help more to understand the context or reason for the question.

Yes your definition helped ScottGem... Thanks a lot.