This must be something to do with Mint, I haven't had that trouble in about 10 years. Yes, I'm also using an MSI motherboard. Jack sensing works perfectly so if I plug headphones in that's where the sound goes, unplug them and sound instantly returns to the speakers.The bad news? I was disappointed in having the exact same experience regarding my audio rear speaker port as I did with my older computer.
And bluetooth still sucks, it always has and it always will.