Hi Alexander,
I moved to new version 26.5 and here my results:
Please see:
Comparison(left-Csv, right-GeoPackage)
image.png (170.3 KB)
Test data(input/output/QGIS)
5330-Csv-GeoPacakge.zip (35.3 KB)
Thanks,...Otherwise the table is saved as a plain attribute table, which QGIS...QGIS reports as “invalid”. Below is a minimal, end‑to‑end example...