This occurs also in Version 14.0.6.1988 (64 bit) - Windows 10 Pro 20H2, Build 19042.1237

Even when opening a second Developer instance, it doesn't appear right away in the task bar. To mitigate this, all other windows have to be minimized and a click into the window of the "missing" Developer instance, makes it available in the task bar.

After all these investigations, the correct Thread title should rather be "PL/SQL Developer not shown in Task-Bar after program start". Unfortunately I can't change that (or didn't found the right button).