I noticed that when you rearrange icons in the dock, they become pinned. How can I stop this from happening?

It's not a feature as it doesn't occur for me. So there's something else at work here. Log into another vanilla admin user account, Set up users, guests, and groups on Mac,  and see if the problem persists. (This tells us if the problem is limited to your user account or is system wide).

This may not be what you'd like to hear, but I can only suggest that you rely less and less on the Dock.

You don't need the Dock to launch an application - and, apparently, you do not use it for that purpose - since your problem is to make sure that applications are not kept in the Dock.

You also don't need the Dock to switch between applications. Use command-tab. No need to reorganize the Dock if you simply don't use it much!

Try this: make sure this checkbox is unchecked in the System/Dock & Menu Bar preference pane:

If this checkbox is not checked toggle it on and off and see if there's any change.

oot into Safe Mode by booting with the Shift key held down and set all of the items you don't want to keep in the Dock to "Remove from Dock". Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

after you unselect "keep in dock", and making sure the app is closed, click and hold the icon and drag it to the middle of the screen. then you will see "remove" appear above the icon. release it and it should disappear.

and i forgot to add, go into SystemPreferences, in the dock & menubar, and un-check show recent applications in dock. then they will appear when open and disappear when you close them. (as long as Keep in Dock is unselected)

ok. i'm mistaken. your video(i looked at it more closely) showed me your issue. that doesn't happen when i try it. i'm sorry.

i think you should report this behaviour to apple: Feedback - Safari - Apple

edited link: Feedback - macOS - Apple

While on the Desktop go to the View ➙ Show View Options menu. There make sure these two settings are set to None:

i just noticed, in your earlier post, "Sketch" is not completely quit. (notice the indicator dot below the icon.)

some apps remain open after clicking the upper left "red dot". with some apps, you need to click and hold it's dock icon until some options pop up. then choose "Quit" to completely close them.

you didn't look closely at my LAST post. i took a screenshot directly from your post. Sketch is OPEN in your screenshot. here it is again. (screenshot of your post)

the black dot under the icon indicates that the app is still open, even if the app's window is closed.