The problem that I described was only due to the fact that I had not adjapted my knowledge of MF4 to the new tools provided in MF6. More tools = more adjustments to make (and understand).
The fact that my saved file was different when reopening it was due the "motion" option that I clicked, before saving it. Contrarely to what I believed, the menu options apply to all the pages and not only to the page someone is working on. It is the same for the "make chapters" option. Unclicking it in one page, unclicks it in the others. It is not a bug, it is the way the program is designed and I did'nt know that. This is not specified in the user manual, but I could ahave guessed that by myself.
Applying that to all my projects made everything work as I wished.
I hope that my errors will help others not to make the same ones.
Soory again, MF6 programmers, and thanks to everyone on this forum for their help.
Pierre.