Game freeze in DetailRenderer::Render() on iOS

We update Unity from 2017.5.2 to 2019.2.1, the terrain detail seems not work stably.
When camera move fast, the game will freeze randomly for seconds. Sometimes it will recover to nomal after several second, sometimes it just stuck forever.
In 2017.5, this will not happen. With terrain detail disabled, stop happen. Reduce other object in scene will reduce the chance of freeze, but still happen.
I tried change the resolution of grass and per patch param, change the grass mesh, redraw the grass texture, still happen.

I use xcode profile, get this:

The red arrows point out the freeze time. Main thread and Gfx thread is full load.


The main thread stack.


The gfx thread stack


Looks like it stuck in waiting job queue, but i can't figure out what it is, or how to walkaround.
Thanks for any help!

 

Update:
When I disable multithread render, the freeze won't happen again. It's a way to walk around, but make cpu load high.
Anyone got a better solution?