When you run the armature symmetrize operator in edit mode, in some cases it mirrors the "custom shape at:" parameter. If I'm not wrong that's the custom_shape_transform property. So, if there was a _R bone in that field, it is mirrored to _L.
I'm attaching a blend file
_Go to Edit mode
_Select Bone_L and Bone_R
_Run armature symmetrize operator
_In pose mode you will see that the bone shapes of both bones have moved to Bone2_R, when the _L bone should have stayed with the Bone2_L address.
In this file this only happens if you select the 2 bones I mentioned. If you select all 4 bones, the bug does not occur.