This is not an Oyster (or contactless) issue, but it is related to travel in London and needs highlighting.
Since March 2016 Swanley in Kent has been placed in London travel zone 8. This coincided with acceptance of Oyster PAYG from March 9th that year. It also meant that day travelcards for zones 1-9 and period travelcards including zone 8 would be valid at Swanley as well. There is, however, a huge problem which means that if you try to buy these products online you may well be overcharged. The anytime day travelcard costs 40p more, the off-peak one is 70p more and the weekly zone 1-8 travelcard costs £3.60 more.
Why does this happen? Historically National Rail ticketing only recognises London zones 1-6 as a destination when selling a travelcard from outside zones 1-6. If additional zones are required then a special route description of “AAA Zones 7-9” indicates the additional validity (where AAA means also available at). This is only available if the origin station is in zones 7-9, or in some special cases just beyond (eg Watford Junction). In all cases the extra zones 7-9 come as standard meaning that you can’t buy to just zones 1-6. When Swanley was added to zone 8 the existing Swanley to zones 1-6 day travelcards gained the “AAA Zones 7-9” route description, but the price of those tickets was already more than the standard versions available everywhere else. The real problem is that fares regulation prevented Southeastern from reducing the cost of those tickets. And since the pandemic started the responsibility for changing fares has passed to the Department for Transport as they now carry the revenue risk.
This is only a problem if the travelcards are purchased away from Swanley. At the station itself the ticket machines are able to sell the correctly priced travelcards and ticket office staff will ensure that you get charged the correct price. But if you purchase online specifying Swanley as the origin then all websites will sell you the wrong products. Rest assured that Swanley is definitely in zone 8 and any travelcard with validity including zone 8 is valid there. Unlike other outboundary travelcards you can make as many journeys between Swanley and zones 1-6 as you want. So if you want to buy a travelcard including Swanley online then ask for one starting from Bushey which is also in zone 8.
Note that there is a completely different problem with Dartford which means that you actually can’t buy the travelcards with Dartford as the origin. The issue is that the route description is always used to indicate “Not Valid on HS1” or “Plus High Speed”. The day travelcard prices are actually correct for zones 1-9, but without the correct route description they will not be accepted at other zones 7-9 stations. Again the workaround is to specify Bushey as the origin station if purchasing online. The weekly travelcard from Dartford to zones 1-6 is 50p less than the zones 1-8 version so unless you need validity to other zone 7-9 stations you’re better off with the original ticket.