Open Event not found?

My app sends diagnostics when people contact me.
Lately, I am seeing this odd message in their logs (doesnt happen on my machines)
It implies the Open Event does not exist, which is ‘obviously’ wrong

Monday, July 29, 2019 Unhandled exception: FunctionNotFoundException Monday, July 29, 2019 RuntimeRaiseException Monday, July 29, 2019 RuntimeRaiseFunctionNotFoundException Monday, July 29, 2019 MyApp.Event_Open%%o<MyApp> Monday, July 29, 2019 _Z29CocoaFinishApplicationStartupv Monday, July 29, 2019 XojoFramework$4452

What might cause this?

From another post…

I have a call in Open to prevent OSX from making my app use tabs for new documents (because Xojo doesnt resize properly when a window becomes a tab)

It used
and I have now replaced that with

It was breaking in compiled app , that had been hardened.
Didn’t break in debug mode since the debug version isnt hardened.