I found the answer. It’s so simple, it elicited a “Oh, FFS!”
After plugging in the converter cable to the USB port, restart Home Assistant.
That’s it, that’s all.
It came about as a result of this thought: after you plug in the converter, HassOS or Ubuntu detect it immediately, but when is Home Assistant informed of it? The simple answer is: not immediately. It detects it on startup.
That was a rather long journey, only to arrive at such an inauspicious destination.