Also reproduced on 64-bit Win10 with just-released 3.9.0.

Note that string search tries to incorporate a number of tricks (pieces of Boyer-Moore, Sunday, etc) to speed searches.  The "skip tables" here are probably computing a 0 by mistake. The file here contains only 2 distinct byte values, and the relatively huge string to search for has only 1, so there's plenty of opportunity for those tricks to get confused ;-)