A little more info: I can reproduce the problem when building the upstream/10.1p1 tag from salsa's ssh-team/openssh.git repository. Bisecting between that and upstream/10.0p1 turns up 289239046 (upstream: Make ssh(1) and sshd(8) set IP QoS (aka IP_TOS, IPV6_TCLASS), 2025-08-18) as the culprit.
(If anybody else wants to bisect, note that you'll need to "cherry-pick -n b7ee13fbbb4eba" to get the guilty commit and some of its neighbors to compile). Scanning upstream commits from GitHub's openssh/openssh-portable repository, it looks like this may have been noticed and fixed upstream already via beae06f56 (upstream: don't reuse c->isatty for signalling that the remote channel, 2025-10-07). And indeed, cherry-picking that patch on top of 10.1p1 seems to make the problem go away for me. -Peff
