It’s possible your brackets within the message might be creating the issue, maybe try escaping these within your message with a back tick before each bracket, e.g.
Show-InstallationWelcome -CloseApps 'notepad=NotePad App better English description `(please save your data first`)' -BlockExecution -AllowDefer -DeferTimes 3 -TopMost $True
I realise your question here is about the Show-InstallationWelcome, But there was a fix for the Show-InstallationPrompt in Version 3.9.0 of the Toolkit for the -TopMost parameter
Show-InstallationPrompt now has a -TopMost parameter. Fixed the function failing to display a window with -NoWait parameter specified.
As I mentioned (in the other thread you posted recently) - v3.9.3 is the latest Toolkit - might be worth giving this a try.