Variable interpolation doesn't work for POST method

[krotorbobr]

I tried to use variables or ad-hoc filters in Grafana Loki panels.
When I use panel single query in a panel, Grafana uses method GET to send data. In that case variable/filter interpolation works as expected.

Example:

• Grafana query:

  {app="app",log_type="log"} 
  | logfmt
  | appsession_id = "$appsession_id"
  | line_format "{{.message}}"

• GET query from network tab in browser:
  
  

When I use panel with multiple query in a panel, Grafana uses method POST to send data. In that case variable/filter interpolation doesn’t work:
Example:

• Grafana query, one of several from panel:

sum by (channel) (
  count_over_time(
    {app="app",log_type="log",project="$project"} 
    | logfmt
    | message="App" [$__interval]
  )
)

• POST query from network tab in browser: