If the object is shared across instances, you should consider changing the code to use a non-static object of type System.Security.Cryptography.ICryptoTransform instead.

As an alternative, you could also look into using ThreadStatic attribute, but make sure you read the initialization remarks on the documentation.

This example demonstrates the dangers of using a static System.Security.Cryptography.ICryptoTransform in a way that generates incorrect results.

A simple fix is to change the _sha field from being a static member to an instance one by removing the static keyword.
  • Microsoft documentation, ThreadStaticAttribute Class.
  • Stack Overflow, Why does SHA1.ComputeHash fail under high load with many threads?.