How to split a string containing “\u001e” in C#

Just to add to what Chris said: when you use a '\' followed by a 'u' in a string in C#, it expects the hexadecimal code (normally 4 digits) for a specific Unicode value to follow it, and the whole value \uNNNN will comprise a single Unicode character in the string.
So if you writhe this:

string details = "Test\u001eA01";
Console.WriteLine($"length: {details.Length}");

It will print "8", being the four characters "Test", the three characters "A01", and the single Unicode character '\u001E' - which is a special code called "RS" or "Record Separator" and is a legacy from the early ASCII based days of computing, and was used to separate rows in a datastream.
If you look at the content of details in the Visual Studio debugger, you will see the string as you typed it, but when printed:

Console.WriteLine($"data: {details}");

You would get:

data: TestA01

Or something similar, as the console (and textboxes!) does not support controls codes other than CR, NL, and TAB: '\r', '\n', and '\t'