This is really weird. For me, the below program works and outputs the HTML that I somewhat expect:

use strict;
use WWW::Mechanize::PhantomJS;
use Data::Dumper;

my $mech = WWW::Mechanize::PhantomJS->new(
    launch_arg => ['ghostdriver/src/main.js' ],
);
 
$mech->get('https://www.racingpost.com/tipping/naps-table/');
 
sleep 5;
 
my $cont= $mech->selector('.tn-napsTable', one => 1 );

my $content=$cont->get_attribute('innerHTML'); 
print $content; 

__END__

<div class="tn-napsTable__header"><div class="tn-napsTable__main"><div
+ role="but
ton" tabindex="0" class="tn-napsTable__cell tn-napsTable__cell_header 
+tn-napsTab
le__napsTipster_header"><!-- react-text: 15 -->Today's naps / Tipster<
+!-- /react
-text --></div><div role="button" tabindex="0" class="tn-napsTable__ce
+ll tn-naps
...
[download]

This is with PhantomJS v 2.1.1 on Windows and WWW::Mechanize::PhantomJS 0.18.