Hey guys. I kinda ran into a similar issue.
I use my own OpenPGP keyscript which is highly improved upon that ("decrypt_gnupg") shipped by the package. One thing that I do is offer optionally feeding the entered passphrase trough argon2 (the standalone tool from the package of the same name) which I copy via copy_exec. Now the problem is that argon2 is statically linked, so there's no libpthread showing up in its ldd, and thus copy_exec doesn't realise it needs to invoke copy_libgcc. Did anyone of your report this issue anywhere else? I mean it's obviously not crypsetup. But I cannot really blame argon2 either, nor can I really blame update- initramfs. Cheers, Chris.