Yes, this seems to happen for me with Start8 as well.
The menu button only appears when a modern app such as Netflix is running.
I think it might be related to dual monitors. If the modern app is started on the second monitor, the Start screen menu corner thing will appear when you hover over the start button on the first monitor.
If the modern app is started on the first monitor, this doesn't happen. It seems to only happen for me if an app opens on the second monitor.
This is using Start8 1.12 and ModernMix 1.05 beta.