Project models are deleted from project file when Processing plugin is disabled

[thunwal]

What is the bug or the crash?

Any models saved to the project file are deleted from the project file when the Processing plugin is disabled and the project save button pressed afterwards. Re-enabling the plugin before saving does not recover the models.

Steps to reproduce the issue

1. Save a QGIS project to an uncompressed QGS file to be able to track content changes.
2. Create any number of models and save them using the "Save model in project" button.
   The models are now shown in the processing toolbox under "Project models" and an asterisk (*) in the title bar indicates the change to the project.
3. Click the save button to write the models to the project file.
   In the project file, an element <projectModels> appears containing the new models.
4. Disable the processing plugin.
   The title bar does not indicate any change to the project.
5. Click the project save button.
   In the project file, the element <projectModels> disappears.
6. Enable the processing plugin.
   The title bar does not indicate any change to the project.
7. Click the project save button.
   In the project file, an empty element <projectModels/> appears.

You may skip step 5 and re-enable the plugin before saving, but the result (step 6) remains the same: The project does no longer contain any model.

Versions

QGIS-Version	3.40.9-Bratislava
QGIS-Codeversion	69fa2ab303
Bibliotheken
Qt-Version	5.15.13
Python-Version	3.12.11
GDAL-Version	3.11.3 — Eganville
PROJ-Version	9.6.2
EPSG-Registraturdatenbankversion	v12.004 (2025-03-02)
GEOS-Version	3.13.1-CAPI-1.19.2
SQLite-Version	3.46.1
PDAL-Version	2.9.0
PostgreSQL-Client-Version	unknown
SpatiaLite-Version	5.1.0
QWT-Version	6.3.0
QScintilla2-Version	2.14.1
BS-Version	Windows 11 Version 2009
Aktive Python-Erweiterungen
linking_relation_editor	v1.4.0
QgisModelBaker	v8.0.4
solocator	2.0.0
db_manager	0.1.20
processing	2.12.99

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

My expectation is that disabling the Processing plugin does not modify the project file.