It requires renaming the parameter to get it re-evaluated. Otherwise, it keeps showing "No Preview documents".

Repro steps:

1. Create a simple pipeline
2. Stage 1 : $match
3. Stage 2: $count - any name as as paramater "", say "movies"

Both stages are properly evaluated and the count stage shows "movies":1065

4. Create new $match stage, $match {}

The stage appears after $count

5. Rearrange the stages for the $count to be the last stage again. Instead of showing 1065, it shows "No Preview documents".

If I go a rename "movies" in the $count stage to "films", it produces the expected result:

"films":1065