Marilyn - Numbers 64 and 66 are registered as follows
64 is ON94510 - Freehold and ON89439 - Leasehold
66 is ON90083 - Freehold and ON89688 - Leasehold
The freeholder of 64 is also the leaseholder of 66. And the freeholder of 66 is the leaseholder of 64.
So I don't think it's a case of a defect but a case of the two maisonettes being deliberately set up in this way with a freehold/leasehold cross over so to speak
You haven't stated whether it's 64 or 66 you are buying but I assume it's 66 as the other maisonette was sold/transferred in 2024 and that registration is still going through.
If I have understood correctly then worth confirming with your conveyancer that you are purchasing the leasehold of 66 and the freehold for 64 and that any purchase will be of the two titles I referred to, namely ON89688 and ON94510
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Public guidance: dealing with land and property - GOV.UK