If Powershell command separator is ; (semicolon), why does "date; dir" make dir output extra details?

As powershell executes statements one-by-one, I think, it applies output formatting of the first statement to all subsequent statements.

As Get-Date returns an object of DateTime type, it gets formatted as list, affecting your 'dir' output.

You can test this assumption by changing return type of Get-Date to string using 'format' option:

date -Format yyyy-MM-dd ; dir

(this will produce default output for 'dir')

Or by changing default output formatting by pipelining it to Format-Table:

 date | Format-Table ; dir

From Out-Default docs:

PowerShell automatically adds Out-Default to the end of every pipeline…

Definitely, a semicolon itself does not suffice for recognizing such a state:

Get-Alias -Name gal; Get-Location

CommandType     Name                                          Version    Source
-----------     ----                                          -------    ------
Alias           gal -> Get-Alias

Drive        : D
Provider     : Microsoft.PowerShell.Core\FileSystem
ProviderPath : D:\PShell
Path         : D:\PShell

Adding Out-Default to the pipeline explicitly solves the problem:

Get-Alias -Name gal | Out-Default; Get-Location

CommandType     Name                                          Version    Source
-----------     ----                                          -------    ------
Alias           gal -> Get-Alias


Path
----
D:\PShell