User access to their points across multiple sites

Wondering if anyone has gotten this to work:
I have set up shared user accounts across multiple sites and would like to be able to view / have points added through any of those sites.

Shared accounts were set up by having all subdomains use the user tables in the main site through settings.php like so:

$db_prefix = array(
"default" => "slave1_", // the prefix for tables that are not shared.
"users" => "master_",
"sessions" => "master_",
"authmap" => "master_",
"profile_fields" => "master_",
"profile_values" => "master_",
);

I have tried adding the "userpoints" and "userpoints_txn" tables to this array for each subdomain, and it seems to be only partially working.
Once I login to a subdomain after pointing to the main site's userpoints tables, the account's points total is shown as 0 and the transaction history is empty. If I login to the subdomain as admin and perform any action (addition or removal of points) to the account, the points total becomes updated to the correct total. However, the points history seems to remain separate from that of the main site. I want the entire history to be visible from all sites.
Furthermore, when logged in as admin, when I view all userpoints totals from any site, the totals are all correct. So it seems that it is partially working, but I feel like I've missed something critical to get it fully working. Any ideas?