Give the best recommendation supported by the evidence you have and label the remaining uncertainty.

A senior interviewer may be testing whether you can make a decision before every detail is resolved, which is common in real client work.

State the direction, give the strongest reasons, and name the one or two facts that could change your view instead of refusing to conclude.

Practice making interim recommendations during cases so you can synthesize without needing perfect completeness. case interview guide.

That sounds like a synthesis test more than a trick. I would give the current answer and flag what is still unknown.