PowerShell module function does not inherit WhatIf
Caller Preference Variables are not propagated from Call-ShowWhatIf to Show-WhatIfOutput. AFAIK this is a known issue with functions called from a module. In this case, it's the $WhatIfPreference that is not being propagated.
Adding this to your Show-WhatIfOutput should resolve the issue:
if (-not $PSBoundParameters.ContainsKey('WhatIf')){ $WhatIfPreference= $PSCmdlet.GetVariableValue('WhatIfPreference')}It checks if the caller (Call-ShowWhatIf) has -WhatIf specified. It does this when -WhatIf is not specified in the function call (Show-WhatIfOutput)
This is a similar post which described the same issue with -Verbose not propagating.
Related links:
PowerShell.org - Script Modules and Variable Scopes
Scripting Guy - Weekend Scripter: Access PowerShell Preference Variables
TechNet - Import Preference variables from the caller of a Script Module function