When updating a file that is stored on an external sftp server the client does not re-upload the file. The metadata is updated however. An easy repro:
synapse add ~/bye.txt --parentId syn5761826
echo bye >> ~/bye.txt #Change file
synapse add ~/bye.txt --parentId syn5761826 #File is not uploaded
Looking through previous clients both v1.4 and v1.5 are broken but 1.2dev2 works.
Doing a full git bisect shows that the regression was introduced at: