Adding time phased actual work to fixed work task with finish no later than constraint

Question

I am using MS Project Pro 2010, no service packs are installed. I have a 40 hour fixed work task, auto scheduling is on, and I've set a finish no later than constraint 4 weeks into the future. I have assigned a single resource with assignment units of 1.0
and Project correctly determines a 5 day duration and schedules the task from the start of the project with the task start date the same as project start date and 8 hrs per day.
I switch to task usage view with the time phased time scale set to weeks (middle) and days (bottom) and add the actual work field. Calculate on edit is selected in project options. When I add 8 hours to day one (which matches the planned work), the planned
work on the last day of the duration changes to 2 hours and the remaining 30 hours is spread at 10 hrs each to the three days in the middle and the duration changes to 4.2 days. Work stays the same as expected.
The behavior I expected is that after I add the 8 hours of actual work nothing would change since remaining work is 32 hours and there are 4 days left in the schedule and the work is already scheduled for 8 hrs/day. When I continue to add actual work of
8 hours to each day, the duration continues to go down until I’m adding 8 hours past the duration. What am I doing wrong? This behavior persists whether or not I select the "move unfinished parts…” options in the project options or not. The only way to
achieve the expected result is to remove the constraint, add the actual work and then reapply the constraint. I assume I have missed an option that needs to be correctly set to achieve the expected (and desired) result. Thanks.

Answer 1

Thanks, Julie.  I thought it was just me.  The reason for the FNLT constraint is that I'm not really scheduling projects but scheduling a single multi-unit resource in a shop environment where many projects are running through at the
same time.  I've set up a single resource with units equal to the number of shop personnel and I'm intentionally forcing overallocations (should there be any) by levelling and by not allowing Project to move my tasks any further than externally mandated
delivery dates.  From that, I can then see where I will need to alter shop release dates, allocate overtime, hire new workers or subcontract the work.  By entering actual work weekly, I can also see where I might have extra hours to use elsewhere
or vice versa.  Unfortunately, a deadline will not prevent Project from moving the tasks beyond the customer provided date when leveling.  This method worked fairly well in Project 2007 even though it's not really what the software was intended for. 
I obviously only use a fraction of Project's capabilities.  There's probably better-suited tools out there but this is the one I have and it actually does a great job at it.  I'm sure I can still make it work in 2010 but it looks like I'll have to
modify my approach.  I'll leave the question open a while longer and see if I get any additional feedback.  If not, I'll mark it answered and move on.  Thanks, again.