MERGE is TRUNCATING data table - which then fluctuates after EVERY refresh (using static tables)

[swise001]

I've noticed this behavior with one of my Power BI Desktop files.  After every refresh (immeidate back-to-back-to-back refreshes), the imported row count varies wildly (randomly going up or down - and seemingly truncating the static source data.

To diagnose the problem I imported all of the table into the model (with no relationships) - and used a COUNTROW to identify the count of each table.  Upon refresh - the source tables all remain constant.  However, the table where MERGES are used varies wildly. 

Then I took the table with merges and duplicated it 5 times (1 instance after every merge).  I found the the changes occured at every instance of a merge.  These merges are left joins (and again none of the source tables are changing size).  

Any one have an idea of why the merges might be truncating the table data?  These are simple left joins (default merge in Power BI).  I've never experienced this before and even built the report back up from scratch - only to see the behavior continue. 

[swise001 · Accepted solution]

Update - after further investigation - it seems to be caused when I merge against a View Table from SQL.  The same issue does not occur when I merge using a standard table (even though both tables are not changing). 

[v-chuncz-msft]

@swise001,

Thanks for sharing this. You may help accept solution. Your contribution is highly appreciated.