This is odd behavior and I am able to reproduce it and have filed a change request (ID 1507740). Another odd thing is that I only see it with png files. With the following test, I get all of the files created except for Testing2.PnG.
Public Sub SaveAsTest()
Dim c As Camera
Set c = ThisApplication.ActiveView.Camera
Call c.SaveAsBitmap("C:\Temp\Testing1.png", 0, 0)
Call c.SaveAsBitmap("C:\Temp\Testing2.PnG", 0, 0)
Call c.SaveAsBitmap("C:\Temp\Testing1.bmp", 0, 0)
Call c.SaveAsBitmap("C:\Temp\Testing2.BmP", 0, 0)
Call c.SaveAsBitmap("C:\Temp\Testing1.jpg", 0, 0)
Call c.SaveAsBitmap("C:\Temp\Testing2.jPg", 0, 0)
End Sub