where are you using console.log to print it ? you should chack console and see what was actually send . It should be resolved correctly
My prerequest:
pm.environment.set("hello","123")
pm.environment.set("hi",JSON.stringify({
"mean": 1710.2180279856818,
"count": {"mean":"{{hello}}"},
'5MinuteRate': 34.74630977619571,
'15MinuteRate': 11.646507524106095
}))
RequestBody
make sure to select raw>json
console: (see whats actually send)