One earbud out is not listening. It’s waiting for the person to finish.

Half-attention is visible from the outside and it communicates something exact. Leaving one in says the conversation is running alongside something more interesting, and the other person can see the calculation being made about them in real time. They will usually shorten what they were going to say. You end up with less of the thing you were half-listening to anyway.

Take both out, or pause what is playing, when somebody starts talking to you. It takes two seconds and it changes the whole exchange. If you genuinely cannot stop, say so plainly and come back to them, which is more respectful than pretending.