T35734 resulted in a change from /64 to /48. However, considering that administrators are able to block IPv6 ranges of /32, but CheckUsers are only able to see /48, the limits should be normalized. This has been discussed a number of times amongst CheckUsers and Stewards, and there was has been wide support for this change.
I'm not a PHP coder, but I think that this simply requires a change of line 64 of CheckUser.php:
'IPv6'=>48,
to:
'IPv6'=>32,