Indeed there are some limitations to alerting in Grafana (there is ongoing work to make them easier and better), i suggest you have a look at this threads :
Bit of a late answer - but for the benefit of others finding this later:
The issue you see in ‘preview alerts’ is that you are using ‘math’ to transform your time series data; rather than ‘evaluating’ it to a true/false condition - what you want here is to use a ‘classic condition’ eg
