Hi Alexander,
I moved to new version 26.5 and here my results:
Please see:
Input
Image.png (68.2 KB)
Output - are bigger than output
Image.png (82.3 KB)
Test data(input/output/QGIS)
5421 Gml-GeoPackage.zip (45.…...5421: Gml to GeoPackage- Conversion succeeded, input and output... content seamlessly. Input image.png (68.2 KB) Output - are bigger...