Properties.encodeToStringMap when a map passed to it return all keys and values with index

[mobinyardim]

I ran the code below and couldn't figure out why the output is behaving this way.
The Code:

import kotlinx.serialization.ExperimentalSerializationApi
import kotlinx.serialization.properties.Properties
import kotlinx.serialization.properties.encodeToStringMap


@OptIn(ExperimentalSerializationApi::class)
fun main(): Unit {
    val map = Properties.encodeToStringMap(mapOf("key" to "value"))
    println(map)
}

Output:
{0=key, 1=value}
My expected output:
{key=value}

I encountered this behavior when I have a Box class with a field named 'content' of generic type. When I specify the type of 'content' as Map and attempt to encode the Box class instance to a map, it behaves as I described above. I believe this behavior is inappropriate.

import kotlinx.serialization.ExperimentalSerializationApi
import kotlinx.serialization.Serializable
import kotlinx.serialization.properties.Properties
import kotlinx.serialization.properties.encodeToStringMap

@Serializable
class Box<out T>(
    val content: T
)

@OptIn(ExperimentalSerializationApi::class)
fun main(): Unit {
    val boxOfMap = Box(mapOf("key" to "value"))
    println(Properties.encodeToStringMap(boxOfMap))
}

output:
{content.0=key, content.1=value}

what I expect:
{content.key=value}