objects in tween not shown after file reopen

Hi,

I made an animation with classic tween, exported and everything was ok. After a few days I wanted to add somthing to that animation. I reopend the file and animated object were not there anymore. But if I remove all the frames of the animated objects, except the frist keyframe, they appear again. Is this a bug or am I missing something - I am brand new to Animate.

Thank you in advance.