I enabled CFGTRACE, but the logs don't show this BYE; the only BYE that appears is after 5 minutes, which is sent by Asterisk to Kamailio. It's confusing because it seems like Kamailio isn't processing the BYE when i make a hangup with my phone.

Screenshot doesn't help more than raw BYE request would.

But mostly you need to look into kamailio service logs. Better with CFGTRACE debugger parameter enabled. It will show you on which step BYE handling was dropped. Also it may show you some meaningful output regarding the reason it wss dropped.