Yan Junze gave these instructions to the Back-faced Woman after deep consideration, first and foremost, he needed to inform his parallel self from the same timeline of the danger the mother and daughter were facing. This was vital to create the conditions for his return.
Furthermore, not allowing his other self to see him was to prevent causing mental shock and confusion, which might lead to unplanned and unprepared actions.
Since his return, the entire timeline had been altered, and once he returned for good, as long as the "Butterfly Effect Stabilization" did its job, even with the changed timeline, everything would wrap up smoothly with minimal impact.
Yan Junze had no doubt about this.
A few moments after he finished speaking, as Yan Junze was about to climb the stairs, he felt the Back-faced Woman pause behind him. He turned around to see her slowing down, seemingly distracted.