Calling function with typed parameter from outside module

Question

I have a Powershell module ExportHelper.psm1 in which I define a custom class Text and a function Add-Text:

class Text {
  [string]$value
  [Format]$format
}

function Add-Text {
  Param([Text]$text)
  echo $text.value
}

It is important to me that the Add-Text function takes a parameter of type Text as I want to use specific properties of this class and it wouldn't be appropriate for someone to call the function with a string parameter.

If I call the Add-Text function from within ExportHelper.psm1 then I see the expected result:

$testText = [Text]@{
    value = "Test text"
    format = ([Format]::Heading1)
}
Add-Text -text $testText

Outputs:
Test text

If I call the Add-Text function from a separate powershell script Playings.ps1 with exactly the same $testText variable constructed in exactly the same way, I instead get an error:

Cannot process argument transformation on parameter 'text'. Cannot convert the "Text" value of type "Text" to type "Text".

If I try echo $testText.GetType().Name then I see $testText is correctly created as a Text object.

I always Import-Module and Remove-Module at each end of my script to make sure I'm using the latest version of ExportHelper.psm1 and indeed I see changes if I modify the Add-Text function.

So my question is: why does Powershell not understand that the $testText variable defined in Playings.ps1 is already of the same type as required by Add-Text? Why does this error only occur when calling the function from outside the module?