Screenshots taken from: https://github.com/alichtman/shallow-backup, although I can only reproduce this on my own repos, so I imagine it'll be the same for you.
With Refined Github enabled:

Without:

OS: macOS 10.14.6
Browser: Firefox 71.0 (64-bit)
Do you have the more-dropdown feature disabled? If not, do you have a console error you can share? I correctly see the Security and Insights tab under More.
Do you have the more-dropdown feature disabled?
I don't remember disabling that feature, as I haven't modified these settings in about half a year and this bug is new, but that did fix the issue.
Thanks :)
You probably disabled it when it had a bug that prevented you from using the repo (it was stuck open)