Thanks for contacting us with your question!
As far as I can see, when the data reaches max (which is 100) the label is still visible. It is hidden when the value is more than max e.g. 101. If you want to be sure that the label is visible, you can set
visibility: visible on ".highcharts-label" class in css, but keep in mind, that it can cause problems such as showing the labels with values more than max value (101 and more) and they can be mispositioned.
Demo: https://jsfiddle.net/BlackLabel/0msno2zp/
I hope you will find it useful
Best regards!
Jakub