The command seems to rely on the registered version-information of the os and not on the real binary-version:

Get-IcingaAgentVersion
 
Name                           Value
----                           -----
Snapshot
Mayor                          2
Major                          2
Fixes                          0
Minor                          15
Full                           2.15.0

(get-item 'C:\Program Files\ICINGA2\sbin\icinga2.exe').VersionInfo

ProductVersion   FileVersion      FileName
--------------   -----------      --------
2.14.3           2.14.3           C:\Program Files\ICINGA2\sbin\icinga2.exe

You can confirm this with an internal "icinga"-check-plugin which also displays a correct value:
Icinga 2 has been running for 4 days, 19 hours, 20 minutes and 41 seconds. Version: v2.14.3

This is a problem when upgrading IfW with update-icinga -name agent or simply checking versions.
Maybe the output should display both versions to indicate a mismatch, e.g. "registered version: 2.15.0 / binary-version: 2.14.3".