Got it. The Alert Manager triggered the 32-bit version of powershell.exe. I needed to set the RemoteSigned execution policy for scripts in the 32-bit environment and change my powershell.exe path.
(in an elevated CMD prompt) C:\Windows\SysWOW64\WindowsPowerShell\v1.0\powershell.exe" set-executionpolicy remotesigned -force
And then change the action to:
C:\Windows\SysWOW64\WindowsPowerShell\v1.0\powershell.exe -Command "C:\scripts\SomeScript.ps1 '${NodeName}'"
I hope this helps somebody!