If I call Get-Command with a list of modules and the command I'm looking for is in a *different* module, it's not found, BUT THAT MODULE IS IMPORTED!
When you call Get-Command with a module name, and the command isn't in that module, you get an error because the command isn't found. However, if you provide more than one module name, then Get-Command will load the module that DOES contain the command:
# No command found, no module loaded accidentally:
Get-Command Get-Acl -Module Microsoft.PowerShell.Management
# No command found, but the Microsoft.PowerShell.Security module gets loaded!
Get-Command Get-Acl -Module Microsoft.PowerShell.Management, Microsoft.PowerShell.Core