When a section is completed at 100% green progress , the first question of the next section shows 100% progress as well. Here is a screenshot of the above scenario
After the first hint/answer attempt in the subsequent section, the progress bar drops to ~1/3 orange starting point, which can be pretty confusing to see all of your progress disappear when you just got a question correct. Here is shown the progress drop after asking for a hint.
Coupled with only a RESTful indication that a new section has begun, it took me a couple days to see what was happening. This is easily reproduced and is common for every section in Week 2 at minimum.