When you create a business process in a new project, the WorkDefinitions.wid file with default work item definitions is created. After that, when you install Rest and Email Work Items in the project settings and reopen the process designer with the previously created process definition, you get each time different Service Tasks on the palette. Once there are just default ones, second time there is only the installed Rest task, third time just the newly installed Email task and fourth time there are both of them. See the attachments.

I think it is probably because the names of the work items overlap with the default ones.