This is a workaround, I don't really like but it works
To allow cloning templates with bind mounts (Useful if you have folders that are shared, so you know that the folder will be accessible in clones), open /usr/share/perl5/PVE/API2/LXC.pm
and find this part of the code (in my current version, it is line 1413)
foreach my $opt (keys %$src_conf) {
next if $opt =~ m/^unused\d+$/;
my $value = $src_conf->{$opt};