It is clearly stated that item keys referenced in formula for calculated items must also exist and 100% match on hosts and cases when the items become not supported are also listed.
https://www.zabbix.com/documentation/3.0/manual/config/items/itemtypes/calculated#configurable_fields

Please add the same relevant information to Aggregate items as well.
https://www.zabbix.com/documentation/3.0/manual/config/items/itemtypes/aggregate

Otherwise it is not clear how to troubleshoot issues when an aggregate item becomes "not supported" with error like:

"No items for key "system.cpu.load[,avg1]" in group "Linux servers"