I see you’ve had a similar problem in September 2018.
Would you be able to make a screenshot of what you see and ideally of the other sections of the release editor as well?
Also helpful could be knowing the web browser you use and if it has been customized, e.g. extensions and settings affecting Javascript or network requests.
It’s possible you are encountering a bug but without more information it’s hard to help you, others had already requested more info back then yet got no reply.