QST: proj_crs_promote_to_3D & derived CRS

[snowman2]

Is it expected that proj_crs_promote_to_3D on a derived CRS returns a non-derived CRS?

Ran into this with pyproj:

>>> from pyproj import CRS
>>> crs = CRS("+proj=ob_tran +o_proj=longlat +o_lat_p=0 +o_lon_p=0 +lon_0=0")
>>> crs.is_derived
1
>>> crs.to_3d().is_derived
0

PROJ API pseudo code:

PJ_CONTEXT* context = proj_context_create();
PJ* pj = proj_create(
    context,
     "+proj=ob_tran +o_proj=longlat +o_lat_p=0 +o_lon_p=0 +lon_0=0 +ellps=WGS84 +no_defs +type=crs"
);
int is_derived = proj_is_derived_crs(context, pj); // returns 1
PJ * pj3d = proj_crs_promote_to_3D(
    context, NULL, pj
)
int is_3d_derived = proj_is_derived_crs(context, pj3d); // returns 0
proj_destroy(pj);
proj_destroy(pj3d);
proj_context_destroy(context);

PROJ Version 8,1
Operating System: Ubuntu 20
Installation method: Conda w/ conda-forge

[snowman2]

Thanks 👍