Re: 2.4Ghz and 5GHz wifi networks
Because it/you assigned the 2.4Ghz channel before the 5Ghz channel when setting up your Macbook's WiFi settings...the fix is pretty easy though. Open network preferences, and then open advanced preferences and remove the 2.4Ghz network from your list of preferred networks. This will make your computer default to the 5Ghz channel...once this is saved, you can later add the 2.4Ghz channel so that it uses it if the 5Ghz channel is ever interrupted and not available.
Given the OS is still in beta, it is also possible that there's a weird buggy setting in the code that is making your WiFi preferences auto load the more wide reaching of the two channels (2.4Ghz will always have a more broad reach vs. 5Ghzs smaller but more powerful reach). 2.4Ghz channel also has a stronger penetration factor, which could come into play.
If you have done this before, there is a slight chance that your router is not producing a strong enough 5Ghz channel for your Macbook Pro to use properly, which is causing it to default to the back up 2.4Ghz channel. If this is the case you can try resetting your network fully to see if the problem still exists...if it does, you may need to try some internal setting changes (including channel change and possibly firmware updates). If these things still don't remedy the issue, there is a possibility the router itself is malfunctioning and would need repair/replacement.