I'm not sure if this will be helpful or not, but we live in a 1970s house with internal walls made from mixed materials in places, and have noticed cracking in places where the materials meet. All the rooms were wallpapered when we moved in and none of the wallpaper was damaged, even the vintage stuff, so I don't think that the movement can be large. The rooms are now painted which highlights any cracks.
An example is around most of our window lintels, which have been boxed in with wood or board before finishing with sand/cement and gypsum top coat. These are part of walls which are made from hollow block with the same backing and gypsum finishing coat. The cracks in these areas are sometimes unnoticeable, and sometimes open slightly depending on how the weather has been or if we have had the heating on a lot. No one has ever been concerned about them, and it was a builder who told us (when plastering over a crack as requested by us), that the house would never be crack free because of this mix of materials in some areas which will expand and contract at different rates to each other.
In the latest room I have redecorated, I have used decorator's caulk in one of the persistent cracks, and while the emulsion doesn't look quite the same on the caulk as it does on the rest of the wall, it does mean that if there is cracking at any point, it is only noticeable until the crack closes again, and then the caulk self heals giving the wall a smooth look. I will be doing this from now on I think as it has proved much more effective than filling with polyfilla or joint compound which doesn't have the flexibility needed.
Sorry if that is no help! If in doubt I think a structural engineer would probably be a good place to start, or a local builder maybe if you know a good one and trust their opinion? They obviously couldn't give you the certainty, but might be able to point you in the right direction if these are caused by something that needs fixing?