In your file, https://github.com/ndunks/fiberhome-AN5506-04F-/blob/397353917fccf82215cf8a377076cfaa5e59d6ce/filesystem/fh/extend/vsftpd.conf
At line 43: idle_session_timeout = 600
This setting itself is a misconfiguration and adding it to your system will not change any system misbehavior.
Root cause: “data_connection_timeout” has the system preset default value, 300. The configuration “idle_session_timeout” has no effect if user does not set “data_connection_timeout” to less than or equal to zero.
Here is the VSFTPD source code to support this. To enable the configuration parameter "idle_session_timeout", user has to explicitly set another configuration parameter "data_connection_timeout" less than 0.
if (tunable_data_connection_timeout > 0)
{vsf_sysutil_set_alarm(tunable_data_connection_timeout);}
else if (tunable_idle_session_timeout > 0)
{vsf_sysutil_clear_alarm();}