Welcome Bertrand in the boat, this problem exists since FMP 7 at least, FM Inc is well aware of it, FM Inc don't care a bit about this data loss prone bug.
My advise, if you've not made to much time investment in Filemaker, ditch it and use another tool
We suffered catastrophic data loss last year over this stupid bug (~hundred thousand+ records). OK we do regular backups, but what a mess... a group of us worked 5pm until 10:30pm to fix the disaster.
As a work-around we no longer rely on table->table imports and instead export to .csv as an intermediate step.
FileMaker prides itself in it's newer, improved ability to connect and share information with external sources. We deal with huge volumes of external data and we can NOT control the field names they send us. And every time we add a field in one of our tables, it maps an additional field from the external source to it (even if the import script step has been disabled!!!). This, of course, destroys the data our Users are trying to put in this field the next time we import. We struggle with it daily and I must stop there ... it'll send me over the edge if I don't.
Thank you for your post. I have added your post to the original report so it is read by our Development and Software Quality Assurance (Testing) departments.
...so they can sit on it for years to come !