Transaction explanations are not being saved

I keep explaining transactions and getting the green tick, only to find it has reverted to a grey tick next time I look, with the transaction still needing to be explained. I have tried this in a private browser in case there is some caching issue. It happens with income, costs, internal transfers and personal transactions and is creating a real headache as I constantly have 130+ transaction that I cannot reduce. As far as I can see, these are not invoices that are already allocated or part-allocated elsewhere. On top of wasting a lot of time, I have failed to submit my return on time because of this.