After one of instance been closed, a new Notepad++ process created and running in the background, and as the screenshot at the original post, the list might increases over than 10 processes in a day and can only be killed one by one by manually or by restart the PC.

I cannot replicate that, even when I switched to “always start in new instance” checked to try to replicate your experience. When I start a new instance (whether it’s by using Windows Explorer to open a file in Notepad++, or whether I move or copy the active tab in one Notepad++ instance into a new instance), a new notepad++.exe shows up in my task manager. But as soon as I exit that instance (File > Exit, or clicking on the window close-X in the upper right), the task is closed and my task manager shows one fewer notepad++.exe in the list.

I see that your original screenshot showed a lot of plugins. You might try seeing if running without plugins (use the -noPlugin option when you launch Notepad++) fixes the problem. If it does, you can narrow it down to see which plugin is causing the problem. You also might try upgrading to v8.4.6.

I usually open a new instance for keeping different tasks in separate windows, so I might have multiple instances at same time and each has multiple files opening. After one of instance been closed, a new Notepad++ process created and running in the background, and as the screenshot at the original post, the list might increases over than 10 processes in a day and can only be killed one by one by manually or by restart the PC.

Please let me know what else information that I could provide. >"<

Many thanks,
N

My notepad++ always get slower and slower after using for a while,

Are you opening each new tab or file in a “new instance” of Notepad++? This would be dependent on having the setting to allow “multi-instance”, see image below and note my setting is “mono-instance”.
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As it is, you may need to provide more information on how you use Notepad++ and if you leave it running if you put your PC to sleep.

Terry