Project saved in QGIS 4.0 loses all MM plugin settings when opened in QGIS 3.44 (backward compatibility broken)

[RastoHu]

Description
When a project is saved and synced using QGIS 4.0 with the Mergin Maps plugin, opening that project in older QGIS, e.g. 3.44, loses all Mergin Maps plugin project settings. Backward compatibility between QGIS 3.44 and 3.22 is unaffected — the issue is specific to projects saved in QGIS 4.0 being opened in older QGIS versions.

Examples of affected settings (non-exhaustive):

• Photo attachments configuration
• Layer order
• Feature filters (MM filtering configuration)
• Project CRS

Environment
[ ] Staging [x] PROD

Application (+ app version, build, operating system)

• QGIS 4.0 (saving machine)
• QGIS 3.44.7-Solothurn (opening machine)
• Mergin Maps Plugin version: TBD
• macOS Tahoe 26.3

Steps to reproduce

1. Open a project in QGIS 4.0 with the Mergin Maps plugin — configure any plugin project settings (layer order, filters, etc.)
2. Save the project and sync via the Mergin Maps plugin
3. Open the same project in QGIS 3.44.7 with the Mergin Maps plugin

Actual results

• All Mergin Maps plugin project settings are lost — the project opens with default/empty plugin configuration
• Project CRS is not set and must be configured again manually

Expected results
All Mergin Maps plugin project settings saved in QGIS 4.0 should be readable and correctly applied when the project is opened in QGIS 3.44. Project CRS should be preserved.

Screenshots or logs

Conclusion
Created by Claude and supervised by a human test analyst.

[leorudczenko]

We have encountered a similar backwards compatibility issue when using the Mergin plugin with QGIS 3.x and QGIS 4.x. In our example, the sync button is disabled, therefore preventing us sharing project data.

The steps I used to reproduce this:

• Create a Mergin project in QGIS 3.x, upload it to Mergin server
• Syncing in QGIS 3.x works as normal
• Open the same project in QGIS 4.x
• Now the sync button is disabled

From my testing, the same issue occurs if you switch the QGIS versions around. E.g. if you make the Mergin project in QGIS 4.x and then open it in QGIS 3.x, the sync button is also disabled. Therefore, it seems like something breaks the Mergin Maps plugin when a project transitions across the major QGIS version change.