Advanced Portfolio Report Anomaly

[nvsoar]

Win7_GnuCash 2.6.3, and Ubuntu 1410_GnuCash 2.6.4;

Peculiar anomaly in Advanced Portfolio Report Basis Column noticed in 
one account;
All correct until 30Jun2009; 31Jul2009 basis is lower than 30Jun2009 
basis (and actual basis) despite a positive reported income for Jun2009.
The constant lower basis persists until the current month.
Rebuilding the account results in no change.  No other similar accounts 
are so affected.
Of the four files containing the data, three are in agreement without 
the above error.
Share balance values are correct for all 300+ entries in the account
The basis anomaly exists when the file is pared to consist of two 
records; an initial balance entered using the 30Jun2009 status data, and 
an income account entry using the 31July2009 statement data.

Looking for possible explanations, trouble-shooting ideas.
nvsoar