I hope it's ok to post this since it was sent to me in a private email from someone at Ilford, but there's nothing particularly private/sensitive in it and it might be useful information for anyone having a similar problem to the OP (as I was):
The issue you are seeing (we call it edge frilling) - is virtually always linked to the paper being washed for too long. FB papers are very fragile and we really only advise washing MGFB Classic for between 30-45 mins.
If its washed for a lot longer than that - as the paper gets very soft, it simply can lead to the emulsion lifting of along the paper edges. It can be triggered too - if the paper edges are in contact with other sheets in a washer (ie if a lot of paper is attempting to be washed in a small sink), or if water is running fast and the paper is swirling around a lot and hitting the side of the washing tank.
Our advice would definitely be to not leave paper to stand in water for anywhere near as long as you are doing. ie 5-6 hours would definitely soften the paper way more than its designed to cope with - especially as your final washing method in an archival washer, will tend to be fairly vigorous/long too.
Ideally we would advise that any prints made, simply go from the fixer tray straight into their final wash method container (ie your archival washer - or a sink) - so they are not having to stand in a waiting tray of water, as it really is likely the time in the waiting water tray, thats causing the problems.
I appreciate it might be difficult for you to do this, so can only suggest try to keep the waiting water tray time as short as possible, and maybe just stick to a max of 30 mins in the archival washer - instead of the hour you are currently giving. (Thats assuming it has quite a good water flow).
I hope you are now able to eliminate it happening - from the above comments.