The empty result is most likely FALSE; verify this by using
var_dump() instead of print_r().

Also provide the return value of posix_get_last_error(), i.e.
run the following script and post it's output:

    <?php
    var_dump(posix_getgrnam("tech"));
    var_dump(posix_get_last_error());
    var_dump(posix_strerror(posix_get_last_error()));
    ?>

You are correct.  The output of the script is:
 
bool(false)
int(34)
string(29) "Numerical result out of range"

but the group "tech" is a valid system group.

34 is ERANGE. Here it means PHP underestimated the amount of space necessary for a buffer to hold all the information for that group - obviously exhausted by that list of members. It could try again with a larger buffer.

Thanks! Just to confirm - it's therefore a bug in PHP since I can't tell it how much buffer to allocate for the call, right?

As requinix already said, what apparently happens is that
sysconf(_SC_GETGR_R_SIZE_MAX) returns a size which is insufficient
(which is allowed by the POSIX spec[1]), and so the following
getgrnam_r() fails with ERANGE[2].  posix_getgrnam() could then
try again with an increased (doubled?) buffer size.

Please try the attached patch (try-again).

[1] <http://pubs.opengroup.org/onlinepubs/9699919799/functions/getgrnam.html>
[2] <https://github.com/php/php-src/blob/php-7.3.0beta2/ext/posix/posix.c#L1079-L1090>

<https://github.com/php/php-src/pull/3464> submitted.