rsync over ssh fails with --files-from

[Alex]

Hi, I've been using rsync on fedora over ssh to sync directories for
decades, but suddenly having a problem with transferring multiple files at
a time to one specific host using --files-from. I can't think of what might
have changed to have caused this. Using rsync to transfer a single file to
this problematic host works successfully. It appears to be related to the
number of files in the --files-from filelist. More than nine and it stalls;
less than nine and it finishes successfully. I'm using the same version of
rsync on both sides.

(Server) Protocol versions: remote=31, negotiated=31
(Client) Protocol versions: remote=31, negotiated=31

When running the following command, it appears to collect the list of files
to be transferred, successfully makes the connection with the remote
server, but then just stalls. Using strace on the remote side shows rsync
appears to be waiting for data.

rsync -avvvv --files-from=/etc/mail/filelist.txt -e 'ssh -i
/root/keys/sync-key-v4' /etc/mail/tmp/ polaris:/var/tmp/one/

sync-key-v4 is a passwordless ed25519 key, but I've tried a handful of
other ed25519 keys.

Could it be related to packet size or some kind of network disparity? It's
not related to the size of the files, as I've tried large and small and it
doesn't matter - if the number of files exceeds 9, it fails.
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