Incorrect height transformation between 3D promoted RGF93 and CH1903+

[hernando]

Example of problem

In cs2cs, converting from RGF93 (EPSG:4171) and CH1903+ (EPSG:4150) goes through ETRS89 (EPSG:4258) according to the output printed with PROJ_DEBUG=2.
If I transform coordinates from ETRS89 to CH1903+ promoted to 3D using projinfo, the height gets converted, but if I transform from RGF93 to CH1903+, the height is not transformed. On the other hand, if I transform from . The height doesn't get converted either if I use RGF93 3D (EPSG:4369)

function wkt2_3d()
{
    projinfo EPSG:$1 --single-line -o WKT2:2019 -q --3d
}

# From RGF93 to CH1903+
$ echo 46 6.6 400 | PROJ_DEBUG=2 cs2cs "$(wkt2_3d 4171)" "$(wkt2_3d 4150)" -d 4
...
Using coordinate operation RGF93 to ETRS89 (1) + Inverse of CH1903+ to ETRS89 (1)
46.0012	6.6008 400.0000

# From RGF93 to CH1903+ pivoting on ETRS89
echo 46 6.6 400 | cs2cs "$(wkt2_3d 4171)" "$(wkt2_3d 4258)" -d 4 | cs2cs "$(wkt2_3d 4258)" "$(wkt2_3d 4150)" -d 4
46.0012	6.6008 347.3557

Problem description

The height should be transformed when converting from RGF93 to CH1903+ with 3D promotion.

Expected Output

$ echo 46 6.6 400 | cs2cs "$(wkt2_3d 4171)" "$(wkt2_3d 4150)" -d 4
46.0012	6.6008 347.3557

Environment Information

• PROJ 7.2
• Ubuntu 18.04

Installation method

• Compiled from source