Stop cleanly and listen to the interruption. Senior interviewers often steer more actively, and the interruption is not automatically negative feedback.

Answer the new question directly, then decide whether the unfinished point still matters. If it does, return to it briefly; if not, let it go.

Do not fight to finish a memorized structure or repeat content the interviewer has already redirected away from.

Your objective is to stay responsive while preserving a clear thread. A short transition such as 'That makes sense, let me address that first' is usually enough. For a related reference, see MBB case interview preparation.

One thing that improved this for me was saying the assumption out loud. It made it much easier to adjust without sounding lost.

I got similar feedback in a mock. The issue was less about the content and more about making the priority explicit.