I see the same thing on my *2016* instance - exactly 6 worktable transactions, that started just a few seconds after the server started up.  If I join to `sys.dm_tran_database_transactions`, I can see that these transactions exist in `tempdb` (database_id = 2), as one might expect for worktables:

```
SELECT 
	dtat.transaction_id,
	dtat.[name],
	dtat.transaction_begin_time,
	dtdt.database_id
FROM sys.dm_tran_active_transactions dtat
	INNER JOIN sys.dm_tran_database_transactions dtdt
		ON dtat.transaction_id = dtdt.transaction_id;
```

[![screenshot of query results showing worktable transaction starting at 04:41:04][1]][1]

This also pretty closely lines up with the log messages indicating that tempdb was being cleared and started up during instance startup:

[![screenshot of error log messages showing tempdb starting up at 04:41:03][2]][2]

I can't find any way through the DMVs to link these "transactions" to a particular session, but I think it's safe to say they are a system process and not something to worry about.

---

Since you mentioned that you didn't see these in `sp_WhoIsActive`, it's worth pointing out that system session's are filtered out of that procedures output by default.  You can see them by passing an extra parameter:

```
EXEC sp_WhoIsActive @show_system_spids = 1;
```

  [1]: https://i.sstatic.net/DXQzf.png
  [2]: https://i.sstatic.net/c16KK.png