After office365 is closed, the xll file is locked

i use office 365 (1809),
open a xll ,close excel,open xll agaiin,close excel,etc...

delete xll ----------->xll is locked by excel 365
wait 15s~30s, i can delete the xll file.

When I closed excel, excel left a process, you can see it in the process manager, you may only delete the xll file until the process exits(waiting for 15s~30s, the process exited).

It seems that the excel document does not have this problem.