As part of MDL-70854, and MDL-80739 and other trackers I've noticed a weird bug where if you render something in a task then it starts off using the web renderer and emits a ton of html junk you don't want on the cli, and then at some point it correctly detects things and swaps midway to the cli renderer.

https://github.com/catalyst/moodle-tool_testtasks/tree/master/classes/task/progress_task.php

clear; php admin/cli/scheduled_task.php --execute='\tool_testtasks\task\progress_task'

This is a problem will using any renderer method and not just a progress bar issue.