"Share Bal" column in Investing Account shows "0" for future transaction for one security?

I typically enter "future" dividends in my various non-retirement Investing Accounts, projecting 3 to 4 months out. 

After entering a "Div" Action the number of shares of that particular stock or fund is usually displayed in the "Share Bal" column on that entry.  I noticed today that future dividends for one particular security - Vanguard GNMA Fund - showed the correct number of shares for two of the four projected dividends but showed "0" shares for the later two projected dividends.  There was no reason for this that I could see; there's no future sell or removed action to cause this and looking at the Account's "Holdings" for the next 4 months continued to show the shares stable at the 8,182.777 shares actually in the account at Vanguard.  Ctrl-Z did not change things.



I don't expect this to be a "real" problem but it does seem odd.  Thoughts about what might cause this?  The only thing unique about this particular security is that it's the only one that I project into the future that pays dividends monthly.  All of the other securities where I make future projections pay quarterly dividends.