Published .mp4 file is cut short

Difficult to answer. But why use Captivate, which is an eLearning authoring tool with focus on interactivity, to create a passive video? Are you talking about a cptx project, or do you try to publish a Video Demo (cpvc) project? If it is the first type, my comment about all lost interactivity is valid, and that is not what Captivate is meant for. You should have to reduce whatever is possible to have a more acceptable file size: audio quality, resolution, quality images, use SVGs instead of bitmap images, avoid embedded videos etc... Not only the duration is important for the file size.

If it is a Video Demo project, Captivate is meant to publish to mp4, try reducing the settings. I mostly try to publish in smaller chunks and stitch them together (I use Premiere Pro) in a dedicated video editor, but didn't have issues to publish even longer files from cpvc projects.