You are not giving much contextual details, but I'm guessing the problem may be due to the fact that the button is only displayed when the Issue is in RESOLVED state (as defined in $g_bug_resolved_status_threshold).

This is not consistent with how $g_allow_reporter_close is documented in the Admin guide, but matches the PHPDoc in config_defaults_inc.php, which states

Allow reporters to close the bugs they reported, after they are marked resolved

Removing the bug_is_resolved( $p_bug->id ) check in access_can_close_bug() function, makes the Close button to be displayed, if the workflow settings allow moving from the Issue's current status to Closed.

This check was added in 2012 (MantisBT master 085ab169, see 0014156), matching the PHPDoc specification.

Now the question is, what is the correct / intended behavior ?

Thank you for your reply.
I understand! It is true that the problem state is not yet resolved.