You sure the plug is in all the way? A not-quite-fully-inserted 3.5mm plug will give you a single channel. Not a new phenomenon, 3.5mm plugs are older than I am.

I’m having this same problem and used same troubleshooting methods.  

Yes! The base of the plug is against the speaker. So assuming that Sonos didn't make the port deeper; it's as far in as possible

Silly question, but you’re certain this plug you’re using is a stereo, and not a mono plug?

Maybe to clarify: My profession is in audio so I know the difference between components .
To answer your question with a straight answer: Yes, it's stereo. I tried different cables too just to be sure, tried different input devices, and tried changing the latency and compression . Currently I'm thinking either the Five Line-In is faulty or I'm missing some stupidly small thing inside the app what I can't seem to figure out.

One never knows, so thank you for the extra clarity. I can certainly remember several situations in the recent past that were caused by that particular issue. 

I’d suggest at this point that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. I’m not 100% sure a failure in the line in electronics will show up, but it will start a conversation I think needs to occur, and I think you’ve gotten all you can from the community. 

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.