As of T112592, we can now see how much is dropped client-side (before it even reaches the server). While the limit is being increased (T112002) considering the fact that nearly every event is dropped, the problem is likely in the schema itself and/or in how UploadWizard uses it. There are too many keys, or too many large keys or values.
It fluctuates between 20% and 100%. Some stretches of time all events are dropped (none validated), other moments very few are dropped.
How is this data used?