InfluxDB notifications not running - influxdb

Question

Create a check. Schedule every 1m offset 8s. CRIT when value is above 20 -- which it is.
Create a notification endpoint: HTTP POST to nc running on http://127.0.0.1:8087 with no authentication.
while true; do cat 201.txt | nc -l -p 8087 -q 1 | dos2unix >> in.txt; echo -e "\n----" >> in.txt ; done
Where 201.txt is
HTTP/1.1 201 Created
Server: netcat
Content-Type: text/plain; charset=UTF-8
Created
Checked working from REST client.
Create a notification rule: run every 2m offset 21s when status is equal to CRIT and message to endpoint.
Nothing happens!
The check has a green tick next to it saying success.
The view history shows a graph with a red horizontal line at the threshold and a blue line for the data. There are two rows in the table below showing runs 90 and 100 minutes ago, and corresponding vertical red lines on the graph.
Suspiciously, when clicking through to the task the red threshold band is not shown on the chart. Clicking to edit shows this definition
import "influxdata/influxdb/monitor"
import "influxdata/influxdb/v1"
data = from(bucket: "zigbee")
|> range(start: -1m)
|> filter(fn: (r) => r["room"] == "Outside")
|> filter(fn: (r) => r["_field"] == "temperature")
|> aggregateWindow(every: 1m, fn: mean, createEmpty: false)
option task = {name: "Test outside 20", every: 1m, offset: 8s}
check = {_check_id: "09b167b262e86000", _check_name: "Test outside 20", _type: "threshold", tags: {}}
crit = (r) => r["temperature"] > 20.0
messageFn = (r) => "Check: ${ r._check_name } is: ${ r._level }. Value is ${r._value}"
data |> v1["fieldsAsCols"]() |> monitor["check"](data: check, messageFn: messageFn, crit: crit)
(I tried lengthening the aggregate window in case it was too small to get any data points, but it hasn't made a difference.)
The notification endpoint history has two entries also from 90 and 100 minuted ago (new data points have been received since then) both with an orange triangle in the SENT column.
The notificaiton rule has a green tick next to it and the same two history entries with orange triangles.
What's going on?!
The orange triangles are probably a red herring because no new ones are appearing -- nothing is triggering. Irritatingly, there appears to be no way to find out what the orange triangles mean.
Just checking through it again there was an orange triangle next to the alert, but when I refreshed the page it went back to a green tick. There are no new rows in the history table.
Update
If I edit the task and then save it then the UI goes to a secret Task page that shows logs.
could not execute task run: runtime error #16:33-16:96: check: failed to evaluate map function: 14:71-14:82: interpolated expression produced a null value
That means my message: apparently there is no such thing as ${r._value}?