This is another issue caused by the VS 11 and the new Find feature with .NET regular expressions change that has been acknowledged and closed as fixed (supposedly in the next build):

Add-Ins: textRanges of EditPoint.FindPattern returns incorrect values for second occurrence
https://connect.microsoft.com/VisualStudio/feedback/details/712348/add-ins-textranges-of-editpoint-findpattern-returns-incorrect-values-for-second-ocurrence