Hello Patrick,
Re-use depth maps is active if:
- current chunk contains the active depth maps instance (in the Workspace pane), to enable storing the depth in the project, it's necessary to switch on "Keep Depth Maps" option in the Advanced preferences tab prior to the depth maps generation procedure.
- in the Build Dense Cloud or Build Mesh dialog (with depth maps source option selected) select the same quality/filtering option that were used to generate the existing depth maps in the project.