The $length function returns the Cartesian value when called from the aggregate function

[pigreco]

What is the bug or the crash?

problem description:
I have a linear layer (testOne) and a table (tableOne) without geometry,
I start the field calc from the table and write the following expression to update the len attribute:

aggregate(
layer:='testOne',
aggregate:='array_agg',
expression:=$length) [0]

the result (126671.709 m) is different from what I would expect (126721.310 m), as it returns the Cartesian value of $length and not the ellipsoid one, in fact if I used $length in the testOne vector I would get the desired value.

data:
https://mega.nz/file/pFQnkYxJ#3zNJR0rzH-a4opip3rgkbJoLrRUlNu7-vwXeiUra_Hs

email:
https://lists.osgeo.org/pipermail/qgis-it-user/2023-December/010471.html

Steps to reproduce the issue

1. create a linear vector with projected SR and add a single line;
2. create a simple table with a single row;
3. When updating the table, use the expression below:

aggregate(
layer:='testOne',
aggregate:='array_agg',
expression:=$length) [0]

Versions

OSGeo4W 64b

Tested on QGIS 3.28.13 LTR Florence and on QGIS 3.34.1 Prizren by OSGeo4W - win 10 PRO 64b

Versione di QGIS
3.28.13-Firenze
Revisione codice QGIS
3b4db4f
Versione Qt
5.15.3
Versione Python
3.9.5
Versione GDAL/OGR
3.8.1
Versione PROJ
9.3.1
Versione database del Registro EPSG
v10.098 (2023-11-24)
Versione GEOS
3.12.1-CAPI-1.18.1
Versione SQLite
3.41.1
Versione PDAL
2.6.0
Versione client PostgreSQL
unknown
Versione SpatiaLite
5.1.0
Versione QWT
6.1.6
Versione QScintilla2
2.13.4
Versione SO
Windows 10 Version 2009

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

the project is set up correctly with ellipsoid, i.e. we know that: 'The length calculated by this function respects both the current project's ellipsoid setting and the distance unit settings'

I attach data and example project.
https://mega.nz/file/pFQnkYxJ#3zNJR0rzH-a4opip3rgkbJoLrRUlNu7-vwXeiUra_Hs