This issue is sometimes intermittent when importing from Monster Builder, or manipulating a creature already in the libraries.

If the creature has a power that does not have an entry in the Range: section, but has text in the Details section. If the text includes the words "melee", "melee basic," "ranged basic," "close burst," "close blast," or "area burst" the text from the Details section will be moved to the Range section. If there is an attack, the Attack calculation will be shoved in the Attack section, and any other hit and effect information will be truncated.

It does not happen when the creature is initially imported or manipulated. Close the application and open the creature again, and the Details section is shoved into the Range section, the attack is in Attack, and the rest of the text is truncated.

I've included a .monster file for import that shows the issue. Change the extension from .txt to .monster to import.

The image shows an example.

Tenebrous Infiltrator.txt