When I switch to material selection, the hair will disappear in 2.8

When I switch to material selection, the hair will disappear, but it actually exists, just invisible, and it will show up when I switch to editing mode. Is this a bug?

Sounds like a bug that you can report in the tracker:
https://developer.blender.org/maniphest/task/edit/form/1/

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The problem has been fixed in the new build.thanks