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