Error in graph when adding transformation


I have multiple queries on the same influxdb. Only one query causes trouble if I add a transformation:
SELECT last(“leistung”) FROM “autogen”.“M800115E14G432” WHERE $timeFilter
GROUP BY time(15m) fill(linear)

All queries do work in the graph but if I add a “total” transformation then the graph shows a line from end to beginning. If I switch to table-visualisation then one (or more) value is added with a timestamp one month in the past:

The resulting graph:

Any ideas?