It could be a number of factors, including what graphics settings you have, or if auto-smooth was set on one of the objects at some time – there’s a lot of variables. The shading menu allows you to override/reset these. Filing a bug report couldn’t hurt any, just in case this is a bug? Here is this link to bug reports: https://projects.blender.org/blender/blender/issues/new?template=.gitea%2fissue_template%2fbug.yaml
