Thanks for reaching out @kingthorin. I'm unable to replicate the behavior you are describing on chrome version 76.0.3809.132 , could you please open a ticket on at https://github.com/contact so we can look into that further.
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Just incase anyone is looking at this in the future. I ended up getting Admin on the repo in question and was then able to set the topics. Unfortunatley prior to that I was also able to see the Topics control which probably isn't right. I was told by someone in support that they'd have the UI team look into it.