Projected Duration of a Task/Issue has gone to 0 and does not match the Planned Duration. This is caused by the Projected Start Date matching the Projected Completion Date. A number of reasons can drive these to be the same, I will list those out that I found on my particular customer case below in the Resolutions section.
There are many factors that can cause the Dates to match. This is largely tied to the allocation against the Task or Issue in question. In most cases, depending on the Duration Type and Task Constraints in use, the Allocation would extend the Projected Completion Date out much further than expected. However, in some cases based on the way these are configured it is possible that this will instead 0 out. Here are the best things to check in those cases, with links to their Supporting Articles:
Projected Completion Date:
Please don't forget to review all of the sub-linked articles within the Task Constraints and Duration Type articles that point to your particular case.
Another consideration that was ultimately the resolution to my particular case is to check on the User's Access Level. In some cases this can point to the assignee being an Access Level that can't commit to work (Reviewer) or the lack of an Access Level at all.