List<headObj> heads = await _repo.GetHeadObjects();
var detailTasks = heads.Select(s => _changeLogRepo.GetDetails(s.Id)
    .ContinueWith(c => new ChangeLogViewModel() {
         Head = s,
         Details = c.Result
 }, TaskContinuationOptions.OnlyOnRanToCompletion));

await Task.WhenAll(detailTasks);

//deadlock here
return detailTasks.Select(s => s.Result);

public async Task<IEnumerable<ItemChangeLog>> GetDetails(int headId)
{
    using(SqlConnection connection = new SqlConnection(_connectionString))
    {
        return await connection.QueryAsync<ItemChangeLog>(@"SELECT [Id]
         ,[Description]
         ,[HeadId]
            FROM [dbo].[ItemChangeLog]
            WHERE HeadId = @headId", new { headId });
    }
}

Dictionary<int, Task<IEnumerable<ItemChangeLog>>> tasks = new Dictionary<int, Task<IEnumerable<ItemChangeLog>>>();
//get details for each head and build the vm
foreach(ItemChangeHead head in heads)
{
   tasks.Add(head.Id, _changeLogRepo.GetDetails(head.Id));
}
await Task.WhenAll(tasks.Values);

return heads.Select(s => new ChangeLogViewModel() {
            Head = s,
            Details = tasks[s.Id].Result
        });