Requests are recorded before decompression:

SessionWorkflow::Impl::_dispatchWork()
...
    TrafficRecorder::get(_serviceContext).observe(session(), _work->in(), _serviceContext);

    _work->decompressRequest();

During replay, requests do need to be modified (e.g., to re-write cursors to match state on the shadow cluster).

However, replaying an uncompressed request which was originally received compressed reduces the similarity of the replayed workload - the server will read more data from the network, and not spend time decompressing.

Investigate tracking compressed vs decompressed, and re-compress requests before submitting them to the shadow cluster, if required.