Once you have successfuly connected to an ftp server, then when it's
time to perform a listing of the servers folder/file(s), then UPNP comes
in. So basically, connecting to a server, is unrelated to listing contents.
By the way, I also have the same model router as you. (Befsr41 rev. 2).
And haven't experienced any problems, when connecting to an ftp server.
Here is an example of UPNP taking place:
[15:27:42] UPNP: Added Port Mapping: Router:40413 -> 10.10.10.10:40413
[15:27:42] UPNP: GetExternalIPAddress returned: "195.141.90.6"
[15:27:42] PORT 195,141,90,6,157,221
[15:27:42] 200 Port command ok.
[15:27:42] Opening data connection IP: 195.141.90.6 PORT: 40413.
[15:27:42] LIST -aL
[15:27:42] 150 Opening ASCII data connection for ls /.
[15:27:43] 755 bytes received successfully. (755 B/s) (00:00:01).
[15:27:43] UPNP: Removed Port Mapping: Router:40413
[15:27:44] 226 Transfer finished successfully. Data connection closed .
As requested, information on an older build of SmartFTP:
https://www.smartftp.com/support/kb/175