using instruments to record display while vsync signals are irregular

When I was using Instruments to test the Display on my phone, I discovered a long-duration frame. Below that frame, there were some gaps in the vsync queue. As I understand it, vsync signals should appear consistently and steadily. How can this behavior be explained?

My phone is iPhone 15 Plus and iOS 18.

using instruments to record display while vsync signals are irregular
 
 
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