On an external MW installation (ver 1.17.0), the parm re max range has been changed as follows:
$wgBlockCIDRLimit = array (
'IPv4' => 8, 'IPv6' => 19, # 2^109 = ~6.5x10^32 addresses
Applied one /8 hard block. Block log and block list are ok. However certain IP's from that range still succeed to register new accounts.
Because of bug 41778 the fields "ipb_range_start" and "ipb_range_end" have been checked. They are of the type "tinyblob".
So it might be that somewhere a routine cannot handle the diff between hi and lo of that range (i.e. 16777216) because a variable has been declared as a 16 bit integer or something like that.
Obviously would never be discovered on WMF wikis because max block range is /16