Using SSRS 2012 SP1 (Version 11.0.3128.0), but was also able to reproduce on SSRS 2008 R2.
Scenario is the following: I have a query that returns KPI values for month/week combinations. The dataset looks something like this: (Note that not every month has the same amount of weeks)
Jan 2013; Week 1; -1
Jan 2013; Week 2; -1
Jan 2013; Week 3; 1
Feb 2013; Week 1; 1
Feb 2013; Week 2; -1
Created a tablix with a row group for the months, and a column group for the weeks. When only displaying the text values returned from the dataset, the results are as expected...cells with no data are empty, i.e. "Feb 2013; Week 3".
When an indicator is inserted into the tablix to display the KPI values in a graphical way, there seems to be a change in the way the data is enumerated.
After the addition of the indicator, the cell for "Feb 2013; Week 3" now contains the column group data for "Jan 2013; Week 3". The KPI value still remains empty/blank.