Since Data Overview to Top Item widget migration in 7.2, the UNIQUE item selection is done by key name instead of item name.

As you can see the item name for both host is the same but the key name parameter varies due to discovery (same SNMP OID root but index varies based on name). 

Since there's two "remote" but only one "Temp" column in the widget I'm guessing the column uniqueness is based on the key instead of the actual name.

I haven't seen this issue with other items, SNMP or else, when tags are used along the default * item after migration: