I haven’t been able to replicate this error on my side, everything works fine with
EasyMorph.Setup.exe /VERYSILENT /SUPPRESSMSGBOXES /NORESTART /DIR="C:\Program Files (x86)\EasyMorph"
Two things to look into, maybe: setup creates icons (links) for the user that's executing this cmdlet; make sure that users match. Second, if I'm not missing something, Execute-Process is not a standard powershell cmdlet, maybe it's implemented in some way that interferes with normal setup process.