Steps to reproduce

function foo {
  [cmdletbinding()]
  Param([parameter(ValueFromPipeline)]$a)
  Begin   {1}
  Process {2}
  End     {3}
}
Write-Host -ForegroundColor Green 'Expected:'
1..3|foo|%{$_} -pv x -ov y |select {$x},{$_},{$y}|out-host

Write-Host -ForegroundColor Green 'Actual:'
1..3|foo       -pv x -ov y |select {$x},{$_},{$y}|out-host

Output:

Expected:

$x $_            $y
-- --            --
 1  1             1
 2  2        {1, 2}
 2  2     {1, 2, 2}
 2  2  {1, 2, 2, 2}
 3  3 {1, 2, 2, 2…}

Actual:

$x $_            $y
-- --            --
 1  1             1
    2        {1, 2}
    2     {1, 2, 2}
    2  {1, 2, 2, 2}
    3 {1, 2, 2, 2…}

Expected behavior

All output produced in any block (Begin, Process, or End) should be assigned to the PipelineVariable as it enters the pipeline. -OutVariable appears to behave correctly, i.e. it captures output that -PipelineVariable does not.

Actual behavior

Only output produced by the first block to exectute, and only on the first execution of that block is assigned to the PipelineVariable.

Examples:

• if a Begin block exists, the output from the Begin block will be assigned, but the output from the Process and End blocks will not.
• if a Begin block does not exists, the output from the first execution of the Process block will be assigned, but the output from subsequent executions of the Process block will not, and the output of the End block will not.

In the wild, on 5.1:

https://stackoverflow.com/questions/45782372/powershell-advanced-function-output-pipelinevariable-doesnt-work

https://www.reddit.com/r/PowerShell/comments/doboko/support_for_pipelinevariable_in_advanced_function/

Environment data

Name                           Value
----                           -----
PSVersion                      7.0.0-preview.5
PSEdition                      Core
GitCommitId                    7.0.0-preview.5
OS                             Microsoft Windows 10.0.18362
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0