The documentation example is different: https://www.boost.org/doc/libs/1_86_0/libs/spirit/doc/html/spirit/qi/reference/operator/alternative.html#spirit.qi.reference.operator.alternative.example

test_parser("Hello", string("Hello") | int_);
test_parser("123", string("Hello") | int_);

In short, a | b ONLY EVER parses b IFF a fails. The reason for this is that a PEG (Parser Expression Grammar) parses left-to-right greedy. This is documented here: https://www.boost.org/doc/libs/1_86_0/libs/spirit/doc/html/spirit/abstracts/parsing_expression_grammar.html (Note the link to Difference parsers).

So in your example string("Hello") will never match, because it would always have matched string("Hell") first. Re-ordering them helps:

test_parser("Hello", qi::string("Hell") | qi::string("Hello")); 
test_parser("Hello", qi::string("Hello") | qi::string("Hell")); 

Prints

Matched: Hell
Matched: Hello

You can also consider qi::symbols which will automatically trie elements in such an order that the longest possible match is found:

Live On Coliru

#include <boost/spirit/include/qi.hpp>
namespace qi = boost::spirit::qi;

template <typename Expr> void test_parser(std::string_view input, Expr const& parser) {
    std::string result;
    if (qi::parse(input.begin(), input.end(), parser, result)) {
        std::cout << "Matched: " << result << std::endl;
    } else {
        std::cout << "Failed to match" << std::endl;
    }
}

int main() {
    test_parser("Hello", qi::string("Hell") | qi::string("Hello")); 
    test_parser("Hello", qi::string("Hello") | qi::string("Hell")); 

    std::cout << "nUsing symbols" << std::endl;
    qi::symbols<char, std::string> sym;
    sym.add("Hello", "Hello");
    sym.add("World", "World");
    sym.add("Hell", "Hell");
    sym.add("Wor", "Wor");

    test_parser("Hell", sym);
    test_parser("Hello", sym);
    test_parser("World", sym);
    test_parser("Wor", sym);
    test_parser("Worst", sym);
}

Printing

Matched: Hell
Matched: Hello

Using symbols
Matched: Hell
Matched: Hello
Matched: World
Matched: Wor
Matched: Wor