Based on this and an additional report with large cookies triggering the same issue, I reproduced and analyzed the issue.
This is happening because haproxy currently lacks support for the H2 CONTINUATION frame, and Chrome 49 and older stupidly cap the frame control size to 1024 byte. I reported this to Willy/the mailing list:
https://www.mail-archive.com/haproxy@formilux.org/msg31134.html