The quiet phone is lying to you.
Your WhatsApp looks fine — no red dot since breakfast. You're not wrong to feel relaxed about it. You're just watching the wrong phone.
By the time you check Instagram, two other places have already replied — one of them at 12:15am. She books there. Nothing was wrong with your WhatsApp; you just can't watch two phones at once. Channel Desk puts every channel on one screen, so no conversation goes quiet just because you weren't looking at that app.