Note: if you are currently using 2.31.0!
There is an addition manual upgrade action required if upgrading from 2.31.0 to later patches (please check the Upgrade notes link below!).
We have an nginx cache running on our play servers, and the cache was causing an empty data response from the server on 2.31.5. I think this is what looked like the bug you saw.
We have cleared the cache now and the behaviour of in event reports seems nominal.
Please can you check and confirm if your original issue is still present on 2.31.5?