when I am working on image "a" and use the color picker tool to sample a color off of image "b", Photoshop switches so that I am now working on image "b". Previous versions (2017) don't do this and it's really annoying, since it creates the extra step of having tp switch back to image "a" every time I want to pull a color off of image "b". I'm assuming this is a bug, since I can't think of any benefit to the way it's working now (sampling from image "b" switches to working on image "b").
That is either a Bug or a new behavior. You should report that as a bug to Adobe. To do thar you should post a bug report on their feedback site. Here is the link Photoshop Family Customer Community Pleas report this bug.