Why does powershell ConvertFrom-Json not work when converting json array and streaming output to ForEach-Object

This is by design, the Windows PowerShell version of ConvertFrom-Json explicitly requests the runtime doesn't immediately enumerate array output (Write-Output $array -NoEnumerate basically).

The reason is that a JSON document with a top-level single-item array like [1] would be indistinguishable from the document 1 if PowerShell's default pipeline enumeration behavior was allowed.

How to work around this behavior?

As you've found, you can work around this by nesting the pipeline that ends with ConvertFrom-Json, using either the grouping operator (...), or either of the subexpression operators $(...) or @(...):

("[1, 2, 3]" | ConvertFrom-Json) | ForEach-Object {Write-Host "Number: $_"}
# or 
@("[1, 2, 3]" | ConvertFrom-Json) | ForEach-Object {Write-Host "Number: $_"}
# or 
$("[1, 2, 3]" | ConvertFrom-Json) | ForEach-Object {Write-Host "Number: $_"}

You can also insert another command into the pipeline - the request to not enumerate array output only affects the immediate downstream command, not the rest of the pipeline:

"[1, 2, 3]" | ConvertFrom-Json | ForEach-Object { $_ } | ForEach-Object {Write-Host "Number: $_"}
#                              ^                       ^
#               This one will not enumerate output     |
#                                              But this one will

Or, if possible, upgrade to the latest version of PowerShell. ConvertFrom-Json in PowerShell 7.x has been updated so that it:

• no longer requests no output enumeration by default
• allows the user to control the behavior with an -NoEnumerate parameter:

# This will work as you expected in PowerShell 7
"[1, 2, 3]" | ConvertFrom-Json | ForEach-Object {Write-Host "Number: $_"}

# This will work the same as Windows PowerShell in PowerShell 7
"[1, 2, 3]" | ConvertFrom-Json -NoEnumerate | ForEach-Object {Write-Host "Number: $_"}