I believe this is NOT a bug but that Index Manager is ‚correcting‘ the colon so that it appears as a colon in the final index and does not act as a special character or regular expression. backslash is inserted automagically before such special characters and regular expressions so that they appear in the final index. Katharina can confirm this, but I think you are okay. (I’ve seen this happen before, too, and am pretty sure that it worked out fine with the backslash.)