Why I can't change position of this application icon in the launcher? I can't move from last icon position

I have installed STS (Spring Tool Suite) IDE on Ubuntu 20.04 and I am finding the following problem with the icon in the launcher.

The problem is that executing this application, it correctly start but the icon is fixed as last element in my launcher left vertical bar:

It is pretty unconfortable. I can't move it another position (as I have done for all the other icons). If I try to drag and drop this icon in another position (between other icons in my launcher bar) it come back to the last position.

I also created an sts.desktop file and I put it into the /home/andrea/.local/share/applications directory, this is its content:

[Desktop Entry]
Type=Application
Name=sts
Comment=Spring Tool Suite
Icon=/home/andrea/DEV-TOOLS/sts-4.11.0.RELEASE/icon.xpm
Exec=/home/andrea/DEV-TOOLS/sts-4.11.0.RELEASE/SpringToolSuite4
Terminal=false
Categories=Development;IDE;Java;
StartupWMClass=STS

After that I created this file if I search "sts" into my application it find it and clicking on the icon it is executed but I still have the same problem: the icon is the last one and I can't change its position