Just started using blender 2.81, I didn't see this issue with 2.80. I do weight painting on one vertex group, and it somehow ends up affecting another vertex group. When I change weights of one bone, it changes another bone's weights. For example for low poly hair, I have bones with weights on, and after doing all the hair bone weights, it completely messes up the main head bone's weights on the hair mesh. What gives? Why is one bone's weight changing another bone's weight?
Red and blue colors means that the area of the mesh is moving or not moving with a specific bone.
Colors that are in between (like green or yellow) means that the area of the mesh is not totally affected by that specific bone. In Other words, it can be controlled by to bones. Yes, the two bones share the same area of the mesh and can contribute to its deformation or motion.
For example, the elbow area of an arm of a character is stretched or compressed using the upper arm bone and the lower arm bone. This happens in nature and should happen in blender as it will not look good if one bone only is responsible for controlling the elbow area.
In your situation, it seems that you need to use more Red and Blue colors by increasing the strength of the brush. Minimize green, orange, yellow, and other colors as much as possible to get the results that you need.
It's not a bug. It's an intended feature to allow more control over what the bones should move. You can upload your .blend file or take screen shots of your issues and add them to the question so that you can get a more specific answer. Good luck with your project and feel free to leave a comment on this answer if there's anything unclear.