Logstash influxdb writes tags with wrong datatype?

I don't know if that top makes sense at all but I'm writing tag fields to influxdb from logstash using

influxdb {
   host => "localhost"
   db => "dynatrace"
   allow_time_override => true
   measurement => "%{[businesstransaction]}"
   use_event_fields_for_data_points => true

This works fine and I'm actually surprised that this also creates tag keys from the tags field in the logstash event:

> show tag keys
name: Order

But when I tried to use the tags in Grafana they didn't work and I finally found out that something is wrong with the tag key/values. They are all listing "true" as value. not the different possible strings.

> show tag values from "Order" with key = y_order_currency
name: Order
key			value
y_order_currency	true

The table:
> select * from "Order"
name: Order
time application businesstransaction responsetime stage2 y_order_currencyy_order_total_price
1467256533592000000 B2C Order 239.75245666503906 true USD 0.10385E3
1467256894845000000 B2C Order 235.5095672607422 true USD 0.5858E3
1467257073363000000 B2C Order 202.9148406982422 true USD 0.54383E3

Any Idea what's wrong here?