Windows 10 ISO mounting bugs

You have narrowed this to Windows installation ISOs on IoT Enterprise LTSC 2021: Explorer hangs, but other applications and other ISO images remain usable. The timing after KB5126657 is useful evidence, but Microsoft’s current release notes do not list this ISO-mounting problem. Test the same ISO through PowerShell using Mount-DiskImage, setting -ImagePath to its full quoted path and adding -PassThru. Microsoft documents this as a supported way to mount an ISO read-only. Record whether the command completes, returns an error, or hangs; then use Get-DiskImage to check its mounted state. Dismount it with Dismount-DiskImage before repeating. A successful command with an Explorer-only hang points the investigation toward the shell; both hanging requires investigating beyond Explorer. Compare one affected installation ISO and one working ISO, recording exact builds and timestamps. Give Microsoft Support these results rather than assuming the update is defective or disabling security software to mount it.