Splash screen in Packr on macOS

Hi,
Build platform: Ubuntu focal fossa 20.0.4.3 LTS
Test platform: macOS Catalina and Big Sur
Packr: release version 4.0.0
build jdk: adoptopenjdk 11.0.x
packed jre: adoptopenjdk 11.0.x

(This refers also to issue #65 :Some ambiguity with vmargs in config.json)

I build my java jar for Linux, Windows and macOS with the splashscreen functionality from java itself, as that is the easiest.
On macOS I use packr v4 recently (I could not make it work with v3, maybe my fault). Before that I created a bundle manually and use a script to start the app with java -jar jExifToolGUI.jar with some vmargs. That showed the splashscreen.
(my app is jExifToolGUI in case you're interested or in case it matters)
Now with pack 4.0.0, even with my builtin java splashscreen, it doesn't work.
If I double-click the the app, it doesn't show the splash screen.
If I do an open jExifToolGUI.app, it doesn't show the splash screen.
If I do an ./jExifToolGUI.app/Contents/MacOS/jexiftoolgui, it doesn't show the splash screen.
If I do a 'java -jar jExifToolGUI.app/Contents/Resources/jExifToolGUI.jar`, I do get the splash screen.

I already created a jexiftoolgui.json with or without a vmargs splash:my_splash_screen and splash:my_splash_screen.png (although the latter with extension should not be needed) and adding the png to my bundle, but that doesn't work either, which is of course in ling with issue #65.
I understand the explanation in issue #65, but why does the Packr binary block my java builtin splash screen?

(It does work on Windows (using launch4j), it works using a .deb and an AppImage, or simply using java -jar ...)

Reproduce the report using jexiftoolgui.json and the three macOS launch paths: double-click, open jExifToolGUI.app, and Contents/MacOS/jexiftoolgui; compare them with running the bundled JAR directly. Inspect the Packr-generated macOS launcher and vmargs handling, using issue #65 as context. Done means the built-in Java splash screen appears through the packaged app launch paths.