Yes, I think the later books still feel as if they are set long before the 1960s.
It is odd because in many ways the entire series could be set in it’s own timeframe (with the exception of the war stories obviously.)
To me Camp and Princess could easily be set in the same decade as the later Swiss books if we didn’t know about the difference in ages. They don’t seem much more remote timewise to me than the later books! I certainly wouldn’t necessarily know they were set in the 1920s and 30s!
The lack of moving into the current time later may have been deliberate. As another poster said the one timeframe that is definitive is that the triplets were born in November 1939. So on that basis in ‘real time’ Prefects can’t be set any later than 1958.
Although, I think EBD deliberately largely stays away from mentioning specific age and time after the war, specifically after arrival in Switzerland. I think that is there is one hint of it being 1950 in one of the island books. Earlier in the series there are various indications of time given. In School at EBD refers to the troubles in Ireland which were mid 1920s. There is reference to a specific Oberamagau in and Jo, which took place in the mid 30s.
Then obviously we know about the outbreak of war in 1939, which doesn’t fit in with those earlier times as if Joey was 12 in 1925 then she had to have been at least 26 by 1939, rather than 19/20 and 56 by end of the end of the series rather than 38 or 39! Although the age she had the triplets at also changes from 20 to 21 to 22 and back and forth as well.
I think by the time she got to Switzerland and was setting a book in each terms she started to fudge the timeline by ignoring real time and dates all together and pretending it was still decades earlier!