My Solution:

After showing slack (format-slack tickbox) on my Gant the line showed my finish date like 4 months after my actual finish which nerfed my critical path.

File -
options -
advanced -
Calculation options for this project -
move end of completed parts after status date back to status date.

This reset the slack back and allowed my CP to work again.

Found this thread which led me to my solution so thought I'd create a login and post. Hope this helps others.

-Ryan