Anyone experimenting the same thing? I thought it was a CSS issue, but even in the Light Theme my footer buttons are disappearing with page loading.
Does the issue still occur when you try safe-mode? If not, then it’s one of your themes / components / plugins.
Are you seeing the same issue here on meta.discourse.org? If so, then the issue might be specific to your browser. You might have a to try an incognito / private window to rule that out.
If none of the above works for you, we need more information like reproduction steps or a link to your site.