Custom report from plugin not showing processed metric in some views

So I have written this working plugin now that shows a custom report. Basically it combines data from an Events and a CustomDimension report.
I added an own ProcessedMetric to it and this metric is calculated correctly and shows fine in the normal report view. But it doesn’t show correctly in the row evolution view and it doesn’t show at all in other exported formats like TSV or JSON.
I had a look at other plugins and I don’t see where this could be added or set.
Does anyone have an idea?