This has been happening to me for a while prior to the latest update. At least a month, as I noticed it when posting about the Lost Caverns of Ixalan spoilers. I thought it was just a feature that Sync hadn't implemented yet. I can edit comments just fine but posts, both on my instance and on other instances, are unable to be edited. I've logged into the web front end and confirmed I can edit posts that way, so it's not due to specific instance rules.