Welcome to Atlassian Community!
There was a bug related to exporting the roadmap a while ago and it was resolved.
I tested here and wasn't able to replicate the same issue.
Can you please let us know if it shows any error while exporting the roadmap?
Is it affecting other users?
Please, try exporting using incognito mode as well.
Also, open the developer tools on your browser and check the console tab to check if it shows any error.