When adding items from different hosts to one application, the item count shows the sum of added items, but in the application item list it shows only corresponding host items. (See attached image).
This is possible to achieve thought application.massadd method.

There is a similar issue with flag fix provided in ZBX-6994 issue, where application.massadd no longer produces validation error when adding item prototypes to application.
It seems like the item prototype can be added, but in reality it skips the adding process.