When I move applications in the bottom bar and open them, they open a new icon and not the one that I moved into the bottom bar. This might sound weird and I would like to show you but I can post pictures yet, sorry.
I recommend you check out Plank. It is very responsive and does what you want. If you’re still using the default layout for XFCE, then you’ll need to remove that bottom panel. Then when you have an application opened that you want to pin to Plank, just right-click it and click on “Keep in Dock”.