Thank you for your answer. I miss the Visiblebuttons property .
1/ I have tried yesterday to create a small project without reproducing the issue. As I have arround 50 grids (40 NextGrid5 / 10 NextGrid6) and more than 100 forms it will take me some time to isolate issue. I'm quite sure I was not getting this issue before starting v6 migration.
2/ Issue is not about adding a page but adding a button or a label on the sheet. When I try to place any component on the sheet it ends on the form. I have to move it back to sheet with the form explorer as cut/paste is not working due to first issue. On my small demo this issue does not appear too. I suspect this to be directly linked with first issue. I will get back to you if I can isolate the issue.