Function App slot health check failing even when its running fine - azure-devops

Question

I have a script thats been used for a long time with no issues for checking the health status of a deployment slot
This is built into the deployment in dev ops
When I deploy the function via my release pipeline its fine, i.e. the slot starts and stays running with no errors
But the health check fails which in turn stops my deployment
The script is a standard ps template as shown below
param (
[string]$URI,
[string]$resultVarName,
[string]$Method = 'GET',
[string]$SuccessTextContent = 'Healthy',
[string]$Retries = 1,
[string]$SecondsDelay = 2,
[string]$TimeoutSec = 120
)
Write-Host "$Method ""$URI"" Retries: $Retries, SecondsDelay $SecondsDelay, TimeoutSec $TimeoutSec";
Function Req {
Param(
[Parameter(Mandatory=$True)]
[hashtable]$Params,
[int]$Retries = 1,
[int]$SecondsDelay = 2
)
$Params.Add('UserAgent', 'azagent powershell task')
$method = $Params['Method']
$url = $Params['Uri']
$cmd = { Write-Host "$method $url..." -NoNewline; Invoke-WebRequest #Params }
$retrycount = 0
$completed = $false
$response = $null
while (-not $completed) {
try {
$response = Invoke-Command $cmd -ArgumentList $Params
if ($response.StatusCode -ne 200) {
throw "Expecting reponse code 200, was: $($response.StatusCode)"
}
$completed = $true
} catch {
Write-Output "$(Get-Date -Format G): Request to $url failed. $_"
if ($retrycount -ge $Retries) {
$completed = $true
$totalCalls = $retrycount + 1
Write-Warning "Request to $url failed $totalCalls times in total."
} else {
Write-Warning "Request to $url failed. Retrying in $SecondsDelay seconds."
Start-Sleep $SecondsDelay
$retrycount++
}
}
}
Write-Host "Status: ($($response.StatusCode))"
return $response
}
$res = Req -Retries $Retries -SecondsDelay $SecondsDelay -Params #{ 'Method'=$Method;'Uri'=$URI;'TimeoutSec'=$TimeoutSec;'UseBasicParsing'=$true }
if($res.StatusCode -ne 200)
{
echo "##vso[task.setvariable variable=$resultVarName]Unhealthy"
Write-Error "Health check validation failure."
}
else
{
echo "##vso[task.setvariable variable=$resultVarName]Healthy"
Write-Host "Health check validation success."
}
Does anyone know how to fix this?
I get the error powershell-scripts\service-health-check.ps1 : [31;1mHealth check validation failure.