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The use case is to have a WebStart configured project from older IDE version and open it in a newer IDE version. At this point, the jnlp-impl.xml is outdated, but everything is still ok and configuration correctly runs the project with WebStart. The problem is triggered when project Properties are opened and then OK is pushed (even without making any changes). Since now, the project is set to <default config> with "Run with Java Web Start" option in Run panel untick, and resets back to this state on every attempt to change the Properties to run with WebStart. The previous Webstart configuration can still be successfully used by selecting it in the combo-box on main IDE toolbar, but it's reset back to the unwanted state every time OK is pushed in the Properties manager. After the semi-automatic jnlp-impl.xml upgrade, the problem is gone. So after issue 200218 is fixed, this problem will be affecting only users, who are intentionally using non-upgraded or modified jnlp-impl.xml files.
as noted, fix of 200218 fixes this usecase.