Java applications gui doesnt look same/ fully load between OSX and Windows?

Hello community,

my friend and I are building a java swing gui project together. We are using git for version controll etc. I have a Macbook Pro from 2016 with BigSur on it, my friend windows 10. We both run on JDK 19.0.1 and have the identical IDE settings.

The Problem: The application he wrote works on his machine, but on my machine only half of the gui content gets loaded and some content appears after I click into the window. When I write an application it works on his Windows machine perfectly fine. When I switch to my windows machine his code works fine. We are using Swing for the GUI and he implements gui components manually while I am using the intellij integrated gui builder. 

The question: Why would his code written on windows does not fully work on osx but the osx code works on windows ?

The application launches and runs perfectly, just the Gui is incomplete. Sometimes it is mostly complete sometimes not.

Thanks.