Hi Alexander,
I moved to new version 26.5 and here my results:
Please see:
Input
image.png (69.6 KB)
Output
image.png (86.6 KB)
Test data(input/output/QGIS)
5427 Gml-TopoJson.7z (35.8 KB)
Thanks,...Bug 5427: Gml to Topojson- Conversion succeeded, input and output...gis read file then during conversion rewrite features with ‘United...