I discovered this while testing the fix for JDK-8248490.

To reproduce this, run the attached test program:

$ java IconifyTest

This test will create an undecorated stage, call setIconified(true) to iconify it, then call setIconified(false) to restore it. After the fix for JDK-8248490 it will iconify correctly, but the call to setIconified(false) fails to restore it.

The window can still be restored using the task bar.

This fails at least as far back as 8u191, and fails in JDK 9 and later, so is not a regression (or at least not a recent one).