Version 6.x added many new features (parameters, themes, etc.) to the module. However, the tags themselves remain backward compatible except for those two parameters:
The D5 version will insert the node content and make it collapsible or collapsed.
The D6 version inserts nothing on these parameters. It only makes the result collapsible or collapsed.
So if you are using D5 and want to keep the collapsible and collapsed look the same, you must add the body keyword as in:
[node:123 body collapsible]
[node:123 body collapsed]