From GGUS ticket:

Hi,

according to the logs it triggers the 'Restore User' action instead which is the default of the form.
The suspend and delete buttons work though. They both have an onclick action to show a confirmation dialog. The java script behind those dialogs changes the action of the form to the one defined by the button (setFormActionFromSubmitButton in jq-sugar.js) before submitting the form. This is not done for the extend membership button, so clicking the button just triggers the default action.
I've done a quick test by adding a javascript function to jq-sugar.js that executes setFormActionFromSubmitButton and adding an onclick attribute to the button which calls that function. After doing that the button started to behave as expected.