In general England / Wales cross border travel with ENCTS passes is subject to local arrangement and very messy.
From the Arriva website;
https://www.arrivabus.co.uk/senior-travel
In general Welsh passes are valid cross border, but English ones are not and hence this offer.
English and Scottish concessionary bus pass holders can save on off-peak Arriva bus travel in Wales.
Use your English or Scottish concessionary pass after 9.30am Monday to Friday and all day at weekends and Bank Holidays to get great discounts on your Arriva bus travel. £4.00 Concessionary Day Ticket (£4.30 from 26 May 2024) £13.50 Concessionary Weekly Ticket (£14.50 from 26 May 2024)
In the Chepstow and Hereford areas on cross border routes, both English and Welsh passes are accepted on cross border journeys and the correct one for non cross border journeys, except the T7 to Bristol (Welsh TrawsCymru route, Welsh passes and may be Bristol issued only). This is courtesy of Herefordshire and Gloucestershire Councils.
Chester Council:-
3. The concessions listed in 1. and 2. are also valid on journeys that cross the Welsh boundary, providing the journey is unbroken and the pass holder boards or alights in the consortium boundary. (probably applies to Chester issued passes only)
TfW;-
Your Concessionary Travel Card is valid on all eligible bus services that operate in Wales. This includes those commencing/terminating at points in England, provided that the cross-boundary journey doesn’t involve a change of bus in England.
This is definitely not complete, but I hope it helps. Ask the operators / councils for confirmation.