>I'd say that the truncation algorithm operates on bytes

This seems likely, as another notable weirdness is that even with full width tabs, where there's plenty of space for at least "𝑼𝒏𝒊𝒄𝒐𝒅𝒆 𝑻𝒆𝒙𝒕..." it still only shows "𝑼𝒏𝒊𝒄𝒐...".