I imported the background image attribute from Maya's deep sequence for mask rendering, and found that the .exr file did not work, but the .rat file worked fine.
I just want to know what the reason is, because it worked fine in 20.0. Is this a software update change?

Ah, I think this may be due to switching to OpenImageIO for .exrs by default in 20.5. Can you file a bug please?

In the meanwhile, you can work around the issue by setting environment variable `HOUDINI_OIIO_EXR` to `0`

Thank! dlee
Your method solved the problem.