RTAO is wrong for my mesh decal(a plane mesh type of decal not deferred decal)

Development Rendering
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image
image1920×1030 178 KB

this is the mesh
image
image1160×853 136 KB

this is the ao which I wanna disable
FYI:I use a masked material with the mesh but the RTAO dont quite take my mask into account.

tried:
disable cast shadow in mesh component setting