ACS simply writes down what it receives from the AS5300, it doesn't do anyting intelligent with it. The problem is the AS5300's are sending this information to ACS differently. This is probably because of how your carrier does it, or different q931 signalling or the like.
On each AS5300 do:
debug tacacs
debug aaa accounting
and you'll be able to see the records that each AS5300 is sending. You'll see they're different, what you do then is up to you. You can talk to your ISP and see why their different, or try changing your AS5300's config to match perfectly, but if you have different signalling methods on your lines then there may be little you can do.
In short though, this is very unlikely to be an ACS problem, since it just records what it receives from the AS5300's.