我有一个 BlogPost 类，我写了一个用来分页读取的方法，如下：

var postQuery = _dbContext.Set<Entities.BlogPost>()
    .AsNoTracking()
    .Where(p => p.OwnerId == OwnerId && p.IsValid);
var list = await postQuery.OrderByDescending(p => p.DateAdded)
    .Skip(( index - 1 ) * size)
    .Take(size)
    .ToListAsync();

这段代码单独运行的时候基本没有发现问题，但是当我并行的执行 UI 自动化测试的时候，他就会抛出下面的异常：

An exception occurred while iterating over the results of a query for context type 'BlogDbContext'.
System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.Collections.Generic.Dictionary`2.Enumerator.MoveNext()
   at System.Linq.Enumerable.WhereEnumerableIterator`1.ToList()
   at Microsoft.EntityFrameworkCore.Query.Expressions.SelectExpression.PushDownSubquery()
   at Microsoft.EntityFrameworkCore.SqlServer.Query.Sql.Internal.SqlServerQuerySqlGenerator.RowNumberPagingExpressionVisitor.VisitSelectExpression(SelectExpression selectExpression)
   at Microsoft.EntityFrameworkCore.SqlServer.Query.Sql.Internal.SqlServerQuerySqlGeneratorFactory.CreateDefault(SelectExpression selectExpression)
   at Microsoft.EntityFrameworkCore.Query.Internal.ShaperCommandContext.GetRelationalCommand(IReadOnlyDictionary`2 parameters)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable`1.AsyncEnumerator.BufferlessMoveNext(DbContext _, Boolean buffer, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable`1.AsyncEnumerator.MoveNext(CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.MoveNext(CancellationToken cancellationToken)

EF Core 版本为 2.2.4。请问为什么会出现“集合被修改”这样的异常呢？