What vers macOS?
Do you use a VPN?
"internet box"? What make/model router is it?
Some NAT Routers have a dynamic/static setting on open ports. The latter holds the port open. The former make it go thru a relatively slow close/open cycle when released.
FWIW, I'm running Transmission 4.0.1 thru PIA. It's got a firm grip on the port; no problems (knock wood).
i'm on macos 13.2.1
no VPN used
SFR internet box (in france)
i used to use this box for years and i always open the port the same way for years. The problem occur only with TM 4.0.1.
i also have another problem : TM 4.0.1 crash... this morning when i woke up it was crashed with the windows : do you want to relaunch/ingore the app... i relaunched but it doesn't do anything i had to reboot my computer (i use a mac mini 2020 as a server with a really clean OS tha use only safari and TM nothing else)