Custom metrics created in Funnel that are based on formulas and used in Google Data Studio are not exported as values to Google Data Studio; instead the actual formula is exported. This allows for correct aggregation of, for example, ratios like CPC.
I'm getting unexpected results on totals when filtering
These metrics have Aggregation Auto in thus should be applied when visualising the value. Ratios like CPC should be calculated on each level they are used to be correct.
There is a known issue with widgets in Google Data Studio that if a filter is used with the widget the metrics formula does not get evaluated on the filtered value, instead it is applied on the values before filtering and typically one of those values is used as a total instead of the correctly aggregated total.
This has been reported as an issue to Google. A work around to try is to create an equivalent calculated metric directly in Google Data Studio as these have been known to get a correct aggregation Auto behaviour.
I'm getting internal errors
There are limits to how long or many formula metrics Google Data Studio can handle. If you have very long formula definitions (ie > 20 fields) this can cause google data studio to freeze. Like described above, a workaround is to make sure Funnel calculates the value by converting it (or parts of it) into a Rules based metric. Thus data studio does not have to handle or calculate these values each time.