Hey there,
we use Jira on premise and experience an odd behavior in comments and descriptions of issues, regarding the link to other issues. An example would make it more clear:
When we reference another issue in an comment or description of a new issue, we simply put the Issue Key (as normally one will do) into the text, but the output (after saving or even on preview of the visual text) will look like this:

This comment wants to reference to the "PRO-27" issue, the "null"-Strings have not been inserted by the person, who created the comment, as seen in the following picture:

As far as i asked my admin of the jira software, he didnt change anything, which my be the cause of this problem. Is there anyone, who already had the same problems and / or knows how to resolve it?