It seems very unlikely that 40px is some sort of min height.
Can you describe in detail how this was made? is it a .swf file or just a movie clip? are you testing in Flash or published to a browser?
Can you post a link to an example?
and any other details you can think of..
This was made with a basic timeline and and a classic tween. I have the height at 39px, doesn't work. Change the height to 41px and it works. This seems to happen only when you are testing in flash.
Tested with a published file in the browser and it seems to work.
This is also happening to me.
Anything below 40px in height with CS5.5 bugs out, unless you actually open up the SWF file directly.
Mine works fine. You might want to make sure your FlashPlayer.exe is updated. Here's the link to download the players and debuggers. On a standard Win7/Vista install you'll find the debugger and player exe in this folder path:
C:\Program Files (x86)\Adobe\Adobe Flash CS5.5\Players
Check the Debug and the Release folders, overwrite the EXEs.
Make sure (Help->Check for Updates) you're running CS5.5 updated (Help->About Flash) at version 184.108.40.2069.
This is DEFINITELY a bug. Thank you TomFletcher for telling me to open the swf file. I was pulling my hair out thinking I was crazy ; )
It's a fixed bug. Update your player/debugger. I can test at 1px height in the CS5.5 IDE just fine.
We are aware of this issue and have a bug open to track this. This is a Mac only issue.
Flash Pro, QE