Loosing the rubber seal into the tank has happened to me twice now, luckily with no damage done, I spotted it both times before most of it had dropped in. The dealer then drained the system etc.
Ironically the earlier tank cap ( which I now have ) seems more robust in this respect - it has a large metal washer holding the rubber in place giving it a lot of support.

As a consequence I dont screw the cap down hard now , as this puts twisting force on the rubber washer aggravating the situation.

Unbelievable this is still happening.