% Complete - doesn't seem to calculate properly, what am I missing?

I'm also encountering the same problem with math errors in Planner premium.

I can reproduce the problem by creating a new task, setting the duration to 1 day, setting the total effort to 8 hours, making the task 50% complete, and then changing the duration to 3 days. I can un-break the task by resetting the duration to 1 day, which reduces the total effort to 4.8 hours, and then resetting the total effort to 8h. Then it seems to work normally again.

New task with duration, effort, and partially complete:

Task after changing duration to 3 days. Completed effort and remaining effort were automatically changed.

Task after reducing duration back to 1 day. Total effort, completed effort, and % complete were changed automatically.

The math is now totally broken for the task. Reducing the % complete to 0, correcting the total effort to 8 hours, and increasing the % complete to 50% give a new incorrect value for completed effort.