The below installs correctly:
PsExec.exe cmd.exe /c C:\Users\alinder\Downloads\setup.exe /s
Yeah, that was me being lazy, I should have included that the regular old wait __Download\setup.exe /s doesn’t work.
No luck with a .bat both with and without wow64 redirection, specifying full path to the exe in a path without spaces. I even moved the .bat to a different directory and called it using the full path and no dice.
Not sure if this a red herring or what, but I notice that after every failed fixlet attempt, there’s an msiexec.exe process left running but not using any resources.