2 stroke Crankseals

I re-built a kettle 3 years ago, part of the re-build was getting the crank
re-furbished by SEP engineering who seemed to do an excellent job.
I have done about 6K miles on the re-built motor and guess what the seals
have gone again manifesting itself in the usual kettle manner by spewing oil
out the gearbox breather all over the bloody place but in particular the
back tyre just to make life that little more interesting.
I was made aware that laying these bikes up for a length of time could
result in the seals hardening then destroying themselves when started. That
hasn't been the case as it has been used often.
I have been known to give it a bit of stick but the limitations of the
frame, suspension mean you can't really tan its arse..
Is it likely to be shite spannerwork on the re-build, bad luck or just a
crap design that causes this failure, I won't go near the unleaded petrol
theory that was proven to be utter bollocks a few years ago.
I have another kettle near completion which leaves me the option of an
engine swap or get it OTR and repair the shagged motor at my leisure.
Just wondered what the 2 stroke guru's that hang about round these parts
make of this as it seems piss poor to only get 6K or is that just the way it
was back then?

Ras
Gt750 dead Gt750 nearly alive 72 Interstate not even looked at

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It's piss poor.