I did raise a ticket on it way back when it first happened shortly after mid April, but having to keep phoning them up and hanging on the line waiting for a tech support person, who invariably didn't have a clue what I was talking about, put me off following it up.
You could ask EE for a replacement SIM to see if that helps (hopefully they don't charge you for it, especially if it turns out to be the problem). Or you could consider swapping the SIM from your tablet into the phone as a test.
So I swapped the sims between the T21 and the G60, and the G60 it had the exact same problems with IPv6, while the T21 worked as it did with its sim. So it does look like the problem is something to do with the G60 and IPv6 and EE's system. Unfortunately I don't have a non-EE sim to try in the phone for 4G connectivity.
I did a little bit of searching online and it does look like EE's IPv6 implementation, and Android, have some issues.