I reported this to the mailing list already, but it is probably better to track this as an open PR, while it stands as it is.
$ sudo zfs send -Rv z at recentSnapshot >a
Assertion failed: (progress), file
% ls -al a
-rw-r--r-- 1 lapo users 694992 21 gen 14:26 a
% zstreamdump <a | fgrep " name =" | wc -l
% zfs list -Hr z | wc -l