Macos LO 2020 - Grid and zooming

Last year I did do some drawings directly in 2019 and before 2018 and before… ( No SU import ). Because I use the mm unit I did set the grid to 10mm with 10 sub units. Easy to do. But the grid snap is horrible, when I snap to the grid and zoom in, the drawing moves away from f.e the main grid line. I did quit drawing in LO and start drawing in another program.

Started yesterday with drawing symbols that must meet a measure and norm.
However, even in 2020, when I’m going to zoom in, the symbol does not stick to the grid. It is shifting.
How can I solve that? Is this a mm unit problem? I am drawing at 1:1

What actually determines the snap point when I have a set of elements in a group? Can I place a point somewhere and mark it as “use this as a snap point”?