[Document Generator] Progress bar jumps from 0-5% to download without showing intermediate progress

After selecting a template and starting the export, the progress bar briefly shows 0% or 5%, then immediately disappears and the 'Download' button is displayed, without showing the actual progress of document generation. 

Steps to reproduce

1. Open a Test/Test Plan/Test Execution (or any supported issue)
2. Click Document Generator
3. Select a template
4. Click Export
5. Observe the progress bar behavior

Actual result
The progress bar shows 0-5%, and progress indicator disappears. The 'Download' button appears immediately.

Expected result
The progress bar displays continuous progress updates (0% -> 100%), and after reaching 100%, the Download button is displayed