Looks like this is an unannounced breaking change in .NET Core 3.0 where we need to change to use EnableBuffering() instead of EnableRewind().
Also as .NET Core 3.0 disallows sync I/O access by default I’ve marked GetRawBody() to AllowSyncIO and have also added GetRawBodyAsync() which should be preferred in .NET Core 3.0.
This change is available from v5.7.1 that’s now available on MyGet.