when to Serve the How to Rent guide

Possession - Section 8 and Section 21

My tenants' started their tenancy on 11th December, the day after the latest up date of the How to Rent Guide was published. However, they signed ( the at that point undated) tenancy agreement on 20th November before the updated guide was published. For the purposes of s21 notices which version should they have received? My letting agent has eventually stated the following in response to my query first made having read your article on 12th December:
When the tenants came in to sign their Agreement and do their Right to rent checks, on the 20th November, I gave them all the correct documents (including the valid Right to Rent Guide) which they signed for. This included the correct version of the How To Rent guide, as long as it is the correct How to Rent Guide at the time the document was given out and the Tenancy Agreement signed, then it is valid. I have also sent them a copy of the new December 2020 Guide via email.

I am concerned that the tenants did not have the updated version of the guide when their tenancy actually started and thus if they know their rights or there is a dispute my ability to serve s21 notice will be lost. I would prefer to know that now and not if and when it comes to a dispute.