Stackoverflow when using async methods

I'm using Dapper in a .net core app to connect to a postgres database using npgsql. I'm using Entity Framework to manage the schema, but querying exclusively with Dapper. I have two repository classes that use the same DbContext. Both are using DbContext.Database.GetDbConnection() to get a reference to the connection. I'm trying to use Dapper's ExecuteScalarAsync<T> but in one of the repository classes, I'm getting a stack overflow. The other works correctly every time. It appears that if I remove the Async and call the synchronous version, the problem goes away. The same problem occurs if I change the method to QueryAsync<T> vs Query<T> - I get a stackoverflow from QueryAsync but not from Query.

Code example:

internal class MyRepository
{
  private readonly MyContext DB;
  private IDbConnection DbConnection => this.DB.Database.GetDbConnection();

  public async Task Insert(MyModel model) {
    var sql = @"
      INSERT INTO public.mytable (someString, someNumber)
      VALUES (@string, @number)
      SELECT currval(pg_get_serial_sequence('mytable', 'idcolumn'))";

    var newId = await DbConnection.ExecuteScalarAsync<int>(
      sql, 
      new { string = model.String, number = model.Number }
    );

    // ...
  }
}

The call to ExecuteScalarAsync results in a stackoverflow exception.

I've seen issues with nested models before, but the model here doesn't have nested object properties, and anyway there's no actual model mapping going on here. I also don't really suspect a connection management issue because I have other working classes that handle their db connections the same way.

Start with the MyRepository example and the DbConnection property, then compare ExecuteScalarAsync and its synchronous counterpart; the report also mentions QueryAsync and Query. Reproduce the stack overflow with the PostgreSQL connection and determine what distinguishes the failing repository from the working one; done means the async call no longer overflows and the behavior is covered by a regression test.