The new version relies on the native browser selection, and thus has less control over the way the selection is drawn. It should be possible to write a plugin that hides the native selection and draws a CodeMirror-5-style backdrop element instead, but I’m not sure when/if someone will tackle that.
When you have multiple selections, secondary selections are maintained and styled by CodeMirror. Maybe it is a good start to its appearance as if all selection are secondary selections.