Here were some suggestions I emailed earlier (although I'm guessing they didn't work for you, or you would have mentioned it!)
Since App.java launches but SimplePaint does not, it seems likely something could be messed up in the "Run Configuration" for SimplePaint. You could compare the "Run Configuration" properties to see what, if anything, is different. (Run menu -> "Run Configurations", and you can view/edit both the SimplePaint and the App run configurations there. )
Maybe try a "Project -> Clean"
Maybe try changing the Java JDK version in the Build Path configuration.
Maybe try "Project -> Update Maven Project" (although this also seems to set your Java version back to 11).
However...
Based on the increasing frequency with which people are having problems using Maven and JavaFX using Eclipse, I am seriously thinking we should all switch to IntelliJ.
So... how about you try installing IntelliJ (free community edition) and then create a new Maven Project (from the openjfx-simple archetype, just like we did in Eclipse), and copy the SimplePaint.java program into your IntelliJ project, and I bet it will run just fine for you there.