std::string and std::string_view have a precise definition for find which specifies what should happen when searching for an empty substring.

http://eel.is/c++draft/string.find
http://eel.is/c++draft/string.view#find
https://en.cppreference.com/w/cpp/string/basic_string/find

This confusion is the reason SERVER-55180 was reverted.
https://github.com/mongodb/mongo/blob/388ce132e9edd64cbfdf677a87ac5044ca3a79ed/src/mongo/db/exec/sbe/vm/vm.cpp#L1978
Changing a value's type from std::string_view to StringData changed the meaning of find.

If a StringData member function has the same name as a std::string or std::string_view member function, it should do exactly the same thing.

StringData::find:
https://github.com/mongodb/mongo/blob/master/src/mongo/base/string_data.h#L283

if (needleSize == 0)
        return 0;

Even if the needleSize is 0, we started looking at position pos so returning zero is incorrect. An empty string is considered found at the leftmost valid position in the search range.

Although StringData::find(substr,pos) takes 2 arguments, none of the tests for StringData ever specify the pos argument, and this function was essentially untested.

Fortunately this bug has no effect unless the pos is given, and very few callers give the pos argument.

There appear to be only two call sites that give the pos argument:

src/mongo/util/net/ssl_util.cpp|44 col 34| size_t headerPosition = blob.find(header, position);
src/mongo/util/net/ssl_util.cpp|54 col 35| size_t trailerPosition = blob.find(trailer, headerPosition);

And these two callers are assured that the header and trailer substrings are not empty because they were constructed locally.