Steps to reproduce:

1. Create 3 items with value of 100
2. Create calculated item with function:
   sum(last_foreach(/hostname/*?[group="Host group"]))

 

Result:
Calculated item gets value of +300 added every item Update interval:

300, 600, 900, 1200 ...

Expected:
Calculated item has a value of 300 every Update interval:

300, 300, 300, 300 ...