string.IndexOf() on Thai Culture , Windows11, 64 bits, .net5,6,7,8, return incorrect index

[enginekit]

using System.Globalization;
CultureInfo.CurrentCulture = new CultureInfo("th-TH");
string line = "/a#name:";
int a = line.IndexOf("a");
int b = line.IndexOf("#");
Console.WriteLine($"a: {a}, b: {b}"); 
//a =1, but b =0, !

---

using System.Globalization;

CultureInfo.CurrentCulture = new CultureInfo("en-us");
string line = "/a#name:";
int a = line.IndexOf("a");
int b = line.IndexOf("#");
Console.WriteLine($"a: {a}, b: {b}"); // Expected output: a: 1, b: 2
//a =1, b =2, correct

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[huoyaoyuan]

Related to #87939