Repro (.NET 5 on ICU):

var ci = CultureInfo.InvariantCulture.CompareInfo;
int idx = ci.IndexOf("\u0600x", "x", CompareOptions.None);
bool isSuffix = ci.IsSuffix("\u0600x", "x", CompareOptions.None);
Console.WriteLine($"idx = {idx}, isSuffix = {isSuffix}");

This sample prints idx = -1, isSuffix = True. These return values are inconsistent with respect to each other. If IsSuffix returns true, then IndexOf must return a non-negative value. It's the contrapositive of the code comment at: