Attribute Table in Layout Manager Font Style Override Doesn't Show Fields

[esnyder-rve]

What is the bug or the crash?

When styling the font of the contents of an attribute table in the Layout Manager, fields are accessible, but not exposed in the "Fields" section of the expression editor for data overrides.

Steps to reproduce the issue

1. Add an attribute table to a layout (with a layer).
2. Go to Fonts and Text Styling
3. Edit the Table Contents Font setting
4. Go to the expression editor to override the Style parameter
5. In the Fields and Values category, only the entry "NULL" exists.
6. You can enter in the field name as normal, and that works.

Versions

QGIS version
3.20.3-Odense
QGIS code revision
495fbae
Qt version
5.15.2
Python version
3.9.5
GDAL/OGR version
3.3.1
PROJ version
8.1.1
EPSG Registry database version
v10.028 (2021-07-07)
GEOS version
3.9.1-CAPI-1.14.2
SQLite version
3.35.2
PDAL version
2.3.0
PostgreSQL client version
13.0
SpatiaLite version
5.0.1
QWT version
6.1.3
QScintilla2 version
2.11.5
OS version
Windows 10 Version 2009
Active Python plugins
db_manager
processing

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

Take a look at the place in which I list my fields, and note that it is barren.

BTW, the expression in there does actually work.