collections.ArrayList Add method producing a string on first Add

Question

I am attempting to mock up a data structure and logic to handle deferred log writing, and I am running into an odd situation. if ([collections.arrayList]$deferredLog = Get-PxDeferredLogItems) { errors when the returned arrayList is only one item, with

Cannot convert the "logged: 07/20/2019 10:56:29" value of type "System.String" to type "System.Collections.ArrayList".

Once there are two items it's fine. Now I know an array of a single item needs to be forced to be an array because PS is sometimes too clever for it's own good, but I thought the Add method of a [collections.arrayList] produced an arrayList even on the first Add. AM I totally misunderstanding, or is my logic wrong somewhere else? I also tried casting the results of the function call like this if ([collections.arrayList]$deferredLog = [collections.arrayList](Get-PxDeferredLogItems)) { but that is even worse, it errors at every call. I also tried initializing the variable to an empty arrayList with Set-PxDeferredLogItems (New-Object collections.arrayList) in Main, rather the n initializing it on the first call of Add-PxDeferredLogItem, but that shows the same behavior, errors until the deferred log has two items. I verified this by changing the Write-Host directly after the erroring line to Write-Host "Process deferred items $($deferredLog.count)", and I get errors till it shows a 2, then everything works as expected.

function Get-PxDeferredLogItems {
    return $script:deferredLogItems
}
function Set-PxDeferredLogItems {
    param (
        [collections.arrayList]$deferredLog
    )
    [collections.arrayList]$script:deferredLogItems = $deferredLog
}

function Add-PxDeferredLogItem {
    param (
        [string]$item
    )
    if (-not $script:deferredLogItems) {
        [collections.arrayList]$script:deferredLogItems = New-Object collections.arrayList
    }
    [void]$script:deferredLogItems.Add($item)
}

function Add-PxLogContent {
    param (
        [string]$string
    )

    function Assert-PxWrite {
        if ((Get-Random -minimum:1 -maximum:4) -gt 1) {
            return $true
        } else {
            return $false
        }
    }

    if ([collections.arrayList]$deferredLog = Get-PxDeferredLogItems) {
        Write-Host "Process deferred items"
        :deferredItemsWrite do {
            if (Assert-PxWrite) {
                Write-Host "$($deferredLog[0]) ($(Get-Date))"
                $deferredLog.RemoveAt(0)
            } else {
                break :deferredItemsWrite
            }
        } while ($deferredLog.count -gt 0)
        if ($deferredLog.count -eq 0) {
            $deferredLogPending = $false
        } else {
            $deferredLogPending = $true
        }
        Set-PxDeferredLogItems $deferredLog
    } else {
        $deferredLogPending = $false
    }

    if (-not $deferredLogPending) {
        if (Assert-PxWrite) {
            Write-Host $string
        } else {
            Add-PxDeferredLogItem $string
        }
    } else {
        Add-PxDeferredLogItem $string
    }
}


### MAIN 
$startTime = Get-Date
$endTime = $startTime + (New-TimeSpan -minutes:2)
Clear-Host

do {
    Add-PxLogContent "logged: $(Get-Date)"
    Start-SLeep -s:5
} while ((Get-Date) -lt $endTime) 

if ($deferredLog = Get-PxDeferredLogItems) {
    foreach ($item in $deferredLog) {
        Write-Host "!$item"
    }
}

EDIT: It seems to be related to passing the arrayList, as this works as expected when there are no functions involved.

Clear-Host

$deferredLogItems = New-Object collections.arrayList
Write-Host "$($deferredLogItems.getType()) $($deferredLogItems.count) $deferredLogItems"
[void]$deferredLogItems.Add('One')
Write-Host "$($deferredLogItems.getType()) $($deferredLogItems.count) $deferredLogItems"
[void]$deferredLogItems.Add('Two')
Write-Host "$($deferredLogItems.getType()) $($deferredLogItems.count) $deferredLogItems"
[void]$deferredLogItems.Add('Three')
Write-Host "$($deferredLogItems.getType()) $($deferredLogItems.count) $deferredLogItems"


function Get-DeferredLogItems {
    if (-not $script:deferredLogItems) {
        [collections.arrayList]$script:deferredLogItems = New-Object collections.arrayList
    }
    [void]$script:deferredLogItems.Add("$($script:deferredLogItems.count + 1)")
    return $script:deferredLogItems
}

$script:deferredLogItems = $localDeferredLogItems = $null
foreach ($i in 1..5) {
    [collections.arrayList]$localDeferredLogItems = Get-DeferredLogItems
    Write-Host "$($localDeferredLogItems.getType()) $($localDeferredLogItems.count) $localDeferredLogItems"
}

EDIT2: Replaced the line # reference with the actual offending line of code, because Confusion.

EDIT3: For what it's worth, forcing the results of Get-DeferredLogItems into an array DOES work. But I still wonder why that's even needed. Why does the Add method produce a correct arrayList at first add but this gets @%#$ed up by PowerShell when passed as a return value?