Inno Setup silently fails to install files - inno-setup

Question

We have an installer created with Inno Setup for installing our VBA add-in and associated files on computers where users do not have administrator privileges.
The destination directory for these files is {userappdata}\Microsoft\AddIns\our-addin-name. In a typical Windows set up this works. However, on some organisations' computers this silently fails: the installer runs, and the dialog is showing the correct destination directory when it is extracting the files, but when you look into the destination directory there is nothing there.
Further comments on one particular case
When the IT administrator had helped the user to run the installation, the installation was successful (presumably he'd run the installer with elevated privileges). However, following a restart, the add-in had gone, and the destination directory {userappdata}\Microsoft\AddIns\our-addin-name is not present.
The user does appear to have write access to {userappdata}\Microsoft\AddIns - the add-in saves data in another directory it creates there - and this was present (i.e. it looks like the system is not trusting files that come from the installer, whilst it is trusting files generated from the add-in). We save data in that directory (as well as a copy in the user's home directory) to get over a problem in some systems where directories appear to be purged in the user's appdata directory between restarts - at least in some systems, this doesn't apply to {userappdata}\Microsoft
Questions/possible solutions
It doesn't seem that the Inno Setup installer is checking whether files are successfully saved in the target directory. What I intend to do is have the installer try to install one of its files into {userappdata}\Microsoft\AddIns, check if it was actually saved there, if not, try another directory - e.g. the user's home directory, and do another check. Then to continue with the installation with whichever path was successful, or otherwise display a message.
I should be able to achieve this with Pascal scripting by extracting one of the files to the temporary directory, attempting to copy that to the target directory and checking to see if the file exists in the target directory.
Does this seem like a reasonable approach?
Minimal reproducible example
I am not able to/do not know how to reproduce the problem on my own machine, and I do not have access to the machines where the problem has been reported. Below is a minimal version of what I am trying to do - and which should fail on problem machines (but does not fail on my own):
#define MyAppName "VBA Addin Install Minimal"
#define MyAppVersion "1.5"
#define MyAppPublisher "My Company, Inc."
#define MyAppURL "http://www.example.com/"
[Setup]
AppId={{E33BF27B-5E1D-4432-B75E-BDA757746A6F}
AppName={#MyAppName}
AppVersion={#MyAppVersion}
AppPublisher={#MyAppPublisher}
AppPublisherURL={#MyAppURL}
AppSupportURL={#MyAppURL}
AppUpdatesURL={#MyAppURL}
DefaultDirName={userappdata}\Microsoft\AddIns\FileInstallTest
DisableDirPage=yes
DefaultGroupName={#MyAppName}
DisableProgramGroupPage=yes
DisableWelcomePage=no
OutputDir=compiled-iss
OutputBaseFilename=setup
Compression=lzma
SolidCompression=yes
PrivilegesRequired=lowest
SetupLogging=yes
Uninstallable=no
[Languages]
Name: "english"; MessagesFile: "compiler:Default.isl"
[Files]
Source: "install-files\show-a-message.ppam"; DestDir: "{userappdata}\Microsoft\AddIns\FileInstallTest"; DestName: "show-a-message.ppam"
Logging
On my machine the installation goes to plan - and the log file indicates that. I have set SetupLogging=yes so that I can get a better idea of what's going on when/if it is reported in the future.