Here’s a quick explanation of what blender does when copying and pasting poses (you might already know this but this is for the benefit of other people reading this question)
If a bone has a ".L" suffix and you copy to the buffer and then paste mirrored. Â It will attempt to find the ".R" bone and flip the relevant channels and paste it to that bone.
It will mirror across the X axis according to the bones parent. (if the armature is not symmetrical when it is built this can paste weirdly)
If the bone doesn’t have a ".L" suffix or it can’t find the ".R" version in the rig it will flip the relevant channels and paste the mirrored pose to the same bone. This is relative to the bones parent.
If you do a normal paste (not mirrored) it will always paste to the same bone (this is useful when copying one pose to a different frame)
What about multiple bones at once?
Once something is copied to the buffer, Blender doesn't care what you have selected or if any bones are hidden. Â If you hit paste or mirrored paste it will apply the pose (or mirrored pose) to the relevant bones.
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In your case, it sounds like a bug. Â I have not had this issue before so I would try to find the circumstances that make it repeatable, record a short video of the issue and then post a bug report.
It might be annoying to narrow down the cause of the issue, but if you can, it stops this being an issue for you and all the other users in the future.