Exchange 2016: setup tried to install wrong CU version

Recently faced an interesting feature on the start at installation of Cumulative Update.

When starting CU setup from the command line, the installation of the current version can start.

An example:
[PS] C:\Windows\system32>cd d:\
[PS] D:\>setup /PrepareSchema
Microsoft Exchange Server 2016 Cumulative Update 6 Unattended Setup

The error lies in the absence of ".\" or exact path to the setup.exe file:
.\setup.exe /PrepareSchema
D:\setup.exe /PrepareSchema

Lets look into the ExchangeSetup.log:

[0] **********************************************
[0] Starting Microsoft Exchange Server 2016 Setup
[0] **********************************************
[0] Local time zone: (UTC+04:00) Moscow, St. Petersburg, Volgograd.
[0] Operating system version: Microsoft Windows NT 6.2.9200.0.
[0] Setup version: 15.1.1034.26.
[0] Logged on user: TEST\administrator.
[0] Command Line Parameter Name='preparead', Value=''.
[0] Command Line Parameter Name='iacceptexchangeserverlicenseterms', Value=''.
[0] Command Line Parameter Name='sourcedir', Value='C:\Program Files\Microsoft\Exchange Server\V15\bin'.
[0] RuntimeAssembly was started with the following command: '/prepareAD /IAcceptExchangeServerLicenseTerms /sourcedir:C:\Program Files\Microsoft\Exchange Server\V15\bin'.
[0] The registry key, HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\v8.0, wasn't found.
[0] The registry key, HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ExchangeServer\v14, wasn't found.
[0] Assembly dll file location is C:\Program Files\Microsoft\Exchange Server\V15\bin\Microsoft.Exchange.Setup.Console.dll
[0] Setup is choosing the domain controller to use

Therefore, be careful.