I can confirm this bug.
You can reconstruct with puppetlabs PC1 repo which has a key including sha-2
d-i apt-setup/local0/key string http://apt.puppetlabs.com/pubkey.gpg
d-i apt-setup/local0/repository string http://apt.puppetlabs.com xenial PC1
d-i apt-setup/local0/comment string Puppetlabs PC1 xenial Repository
d-i apt-setup/local0/source boolean true
Who the hell broke the preseed part of Xenial installer?... this is only one
bug of many many others
With Trusty preseed was working quite well...
I'm asking myself why I'm commenting this bug... seems like nobody
really cares. This issue has been added 2016-03-04...
Xenial is still not ready for business use, even in .1 release...
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Xenial preseed fails to load key for 3rd party repo with apt-
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