A workaround that works:

addpath("C:\example)
fid = fopen ("C:\example\foo.m", "wt");
fputs (fid, "function foo ()\n disp (\"This is function foo.\");\nendfunction\n");
fclose (fid);
addpath("C:\example)
fn = str2func ("foo");
fn ()

This works:

fid = fopen ("foo.m", "wt");
fputs (fid, "function foo ()\n  disp (\"This is function foo.\");\nendfunction\n");
fclose (fid);
fn = str2func ("foo");
fn ()

This doesn't:

addpath("C:\example)
fid = fopen ("C:\example\foo.m", "wt");
fputs (fid, "function foo ()\n  disp (\"This is function foo.\");\nendfunction\n");
fclose (fid);
fn = str2func ("foo");
fn ()

That is because Octave doesn't refresh the functions in others directories, only the current path.

This is something that rehash would fix... right?

This fails too:

fid = fopen ("foo.m", "wt");
fputs (fid, "function foo ()\n  disp (\"This is function foo.\");\nendfunction\n");
fclose (fid);
rehash()
fn = str2func ("foo");
fn ()

Using ignore_function_time_stamp doesn't fix it:

ignore_function_time_stamp("none")
fid = fopen ("foo.m", "wt");
fputs (fid, "function foo ()\n  disp (\"This is function foo.\");\nendfunction\n");
fclose (fid);
fn = str2func ("foo");
fn ()

If I wait a while (like 1 min or something like that), I can do the str2func and works fine.

(Windows, no packages installed, octave 4 clean)

Maybe #35771 is related.