This is a bug in our android app that happens if you move the trees below any slice groups or below the "Add a monetary account" button.
It should be fixed in the next release of the Android app.
In the meantime, if you delete and reinstall the bunq app that will fix it for your case 😊
And you can always still move the trees to the bottom of your Monetary accounts, just not below any slice groups or the "add account" button.
Thanks for the report!