Mine is older now (18) but I remember feeling exactly like you do - it seemed to happen almost overnight, disappearing off upstairs, monosyllabic, eye-rolling and all of that.
It really hurts doesn't it, and it's really worrying. The only reassurance I can offer is that they ARE ok and do come back round. With phones and other tech their friends are with them, albeit virtually, they're constantly in contact, just not physically.
Mine even used to often end up eating in their room as well. I think it becomes much more part of their safe space. I was a single parent and teen was the only one left at home so it was very weird. I'd wander round feeling the emptiness and the teemed happy as Larry squirrelled away!
The middle ground I found worked was to join them in their space - not just barge in obviously but arrange something - we'd get a pizza delivered, or make a favourite easy dinner and watch a movie (teen still usually on their phone!) or play cards and just hang out. They opened up a lot more and we had some great chats.
If you can, try and embrace it - we bought a few bits for the bedroom like LED lights, more cushions and blankets, a little table to eat at and just let them make their 'nest'.
Must be hard with younger ones who probably miss their older sibling, but it will be ok.