Behavior got a bit worse after the recent Windows 10 2004 Update with PL/SQL Developer 14.0.1.1965:

Even if I start the Developer manually from the Startmenu (no matter if attached or from app-list) the task bar entry generally disappears after "splash screen" is gone. It looks like the initial task bar entry IS the "splash screen" entry, which is supposed to disappear after successfully starting the program.

Still the task bar entry of the main application is not visible until it is brought to foreground with Ctrl+Tab. Behavior at auto start is still the same.