Is there any video? That's really what it would take to conclusively identify it.
A video that included an options key press (or whatever PC equivalent is) that showed mode, and later on a hollow square.
Human error is so possible here. Just today I went into solo instead of open accidentally to try to engage a pre-arranged 1v1, and took at least 60 seconds to realize I left my brain in the jar. And I'm a dirty pirate (being called a ganker in another thread RN ugh don't even get me started on that nonsense ugh ugh ugh) who should know better
60 seconds is more than enough time for a rapid murderhobo to do their thing.
I have on more than a few occasions seen [] in solo mode. Actually logged out to check.