"How many a man has committed himself on a short acquaintance, and rued it all the rest of his life!" (Chapter 43) Jane must have assumed Frank regretted their engagement. She pointed out in reply that only weak characters would suffer such an acqaintance to affect them for ever - a clear suggestion that rash engagements may be ended, and perhaps even, with what we have learned since, a warning that theirs might be. Frank reacts by asking Emma to find him a wife and Emma agrees. After that Jane can hardly be blamed if she assumed that he wanted out of their engagement, and that she would need to think of a future without him. His sudden departure for Richmond, as you say, might well have appeared to Jane to confirm her fears. No doubt we will learn exactly what happened if Frank ever gets round to writing the explanatory letter he has promised to send to Mrs Weston.