Ok... further to this. Wrote a small application that allowed me to track down EXACTLY which assignments were causing these issues (we had quite a few) and then by progressively altering data associated with the assignment and Task in Project and then RE-PUBLISHING were able to finally correct the DBNULL issue in the WASSN_ACTUALS_PENDING
NOTE: all the affected Assignments were 100% complete and it is suspected that this combined with some sort of data was preventing them from being part of the full publish? This is a supposition only but a successful one for us. The initial cause of the issue is NOT known at this time.