It shouldn't do that, you hopefully did something wrong. You could try clicking on the symbol on the stage and seeing what it is in Properties. If it has changed to MovieClip or Button, just change it back to Graphic.
open a new fla
create a graphic symbol (insert>new symbol>graphic>ok).
put something on stage like a rectangle.
return to the main timeline
drag your graphic from the library to the stage
insert a frame at about frame 20 in the only existing layer
right click that frame and click create motion tween.
I click on it once and it tells me it is a Group but I double click on it and it tells me it is a Graphic Symbol
I was able to open a new fla and motion tween a rectangle with no problems.
If it's grouped for no reason, ungroup it.
Thank you, that was the problem. I knew it had to be simple.