Starting with version 8.10.1, any code that modifies the rotation or rotationQuaternion property of a target camera when the scene is in right-handed system (i.e., scene.useRightHandedSystem = true) will behave exactly 180° flipped on the Y axis than before. This is due to a bug with the target camera code that fails to take the handedness of the scene into account. See RH camera fixes by bghgary · Pull Request #16691 · BabylonJS/Babylon.js for details.
For additional back story, we ran into this problem because we were trying to fix a camera issue with the glTF exporter. After a lot of investigation from both @alexchuber and myself, we discovered this problem in the code.
Please shout if this causes trouble for your scenarios.