Hi,
Thanks for the details. So I looked into it and I have the same behavior on my end.
HikaShop has a user synchronization plugin, which has, among other things, the capability to add the user in HikaShop when a user is created from Joomla or from another extension. And since the HikaShop registration process adds the Joomla user before it then adds the HikaShop user (so that it can user the id of the Joomla user in the HikaShop user entry as reference), the user synchronization plugin would add the user in HikaShop while Joomla would add the user in Joomla, undercutting the registration process.
This would usually not be a problem since it just means that the saving done by the registration process for the HikaShop user data would end up as an update, and not a creation.
So that's why your mass action would be empty. The "after a user is created" event would actually be before the user data would be saved by the registration process.
So I've made a patch to the mechanism so that the user synchronization plugin will skip the synchronization if the Joomla user creation happens from HikaShop's registration process. It then now properly triggers the "after a user is created" and not the "after a user is updated" when saving the HikaShop user data in the database during the registration process.
To get the patch, download the install package of HikaShop on our website and install it on your website and that mass action should then work fine, like it does on my end.