I haven't had any problems w/ coins expiring.
I recently got an email that said my 1167 coins would expire on Feb 1st. I checked my purchase history and my last purchase w/ coins was almost 1 year ago. So I purchased something for 199 coins. I checked my balance today and I still have 968 coins. Most of these coins are still left over from the May 2014 GTA deal.
System seems to be working for me. Not sure why it's not working according to policy for everyone.