Since version 2.1.5, custom tooltips show [object Object] instead of some of the tooltip data.
Here is a sample which shows the issue: http://codepen.io/anon/pen/grZwmE
This code was taken from your custom tooltip example which is now broken.
#2775 is a similar issue which exists since 2.1.5. It was fixed in 2.1.6, however this one is still present in 2.2.0 RC2.