Add-Member ScriptProperty deduce empty array or single item array to no array item [duplicate]

$obj = New-Object -Type PSObject
$obj | Add-Member -Type ScriptProperty -Name ArrayMember {"" -split ","}
$obj.ArrayMember.GetType().Name

Output: String

$obj = New-Object -Type PSObject
$obj | Add-Member -Type ScriptProperty -Name ArrayMember {"1" -split ","}
$obj.ArrayMember.TetType().Name

Output: String

$obj = New-Object -Type PSObject
$obj | Add-Member -Type ScriptProperty -Name ArrayMember  {"1,2" -split ","}
$obj.ArrayMember.GetType().Name

Output: Object[]

I want to directly use $obj.ArrayMember += 3 to append 3 into the array, but if it return a string, like "1", the result will be 13, but I expected [1,3].