[Qgis-user] Beginner question about "Add geometry attributes" with Sperical ellipsoid

[Hilbert Set]

Hello,
I am using QGIS version 3.12.1-București QGIS code revision121cc00ff00 on
Windows 64 bit.

In the properties window for the project, Project > Project Properties >
General > Measurements > Ellipsoid (for distance and are calculations), I
set the value "WGS 84 (EPSG:7030)"

Then I try to add geometry attributes to a line layer, as explained at page:
https://docs.qgis.org/testing/en/docs/user_manual/processing_algs/qgis/vectorgeometry.html#id212

Vector > Geometry Tools > Add Geometry Attributes
I set: calculate using Ellipsoidal

It works fine: I get a new layer which includes an attribute "length" with
the length of the lines (in meters).

Now I want to repeat the computation of the length, but with reference to a
different ellipsoid: In the properties window for the project, under
Project > Project Properties > General > Measurements > Ellipsoid (for
distance and are calculations), I set the value "Sphere GRS 1980 Mean
Radius (ESRI:107047)"

This time when I try to add geometry values to the original layer, in the
same way as descirbed before, it looks to work fine, I get no error message
and get a new "length" attribute, but its value is "null".

If I set again a different ellispoid in Project > Project Properties >
General > Measurement > Ellipsoid (for distance and are calculations),
provided it is not a spherical one, it works fine again, with numerical
value of distance computed instead of "null".

So, my question is: how can I get QGIS to compute the length of the lines
using the spherical ellipsoid?

Thanks
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