The original bug is filed here, PowerShell v5.1
I can reproduce it with the latest powershell-6.0.0-alpha.14-win7-x86.

Here is the description of the issue and steps to reproduce.

Create a directory C:\TEMP\HelpAlias (we assume it is not in the path yet).

Create a script Test-Me.ps1 in it:

<#
.Synopsis
    Test help

.Description
    Test help
#>

Open a PowerShell console and invoke the following commands:

    0> Set-Alias zz C:\TEMP\HelpAlias\Test-Me.ps1
    0> Get-Help zz

        Name          : zz
        Category      : Alias
        Synopsis      : Test-Me.ps1
        Component     :
        Role          :
        Functionality :

    0> $env:path = $env:path + ';C:\TEMP\HelpAlias'
    0> Get-Help zz

        NAME
            C:\TEMP\HelpAlias\Test-Me.ps1

        SYNOPSIS
            Test help

        SYNTAX
            C:\TEMP\HelpAlias\Test-Me.ps1 [<CommonParameters>]

        DESCRIPTION
            Test help

Expected behavior

Get-Help works for properly defined aliases to scripts regardless of
their location, i.e. included in the path or not. In other words, the
result is the same as if Get-Help is invoked with the script path
defined by the alias.

Actual behavior

We can see that the alias zz is not resolved by Get-Help when the script is
not the path. Then it is resolved fine after adding the directory to the path
and Get-Help zz gets the expected help.