The EU signed a trade treaty with Canada and assured us that everything from that point forward was just a matter of formalities. Then Germany decided there was a clause in there they didn't like, and that no the treaty was not going forward, signed by the EU or not. Thereafter followed mad last minute top level negotiations directly between Canada and Germany to find something acceptable to both sides, with the EU then being informed of what had been decided on and the changes were incorporated into the treaty.
Get used to it, it's how the world works and the UK will have to be prepared for this in dealing with anyone else. No treaty text is final until all parities have ratified it in their parliaments. It's not at all unusual for one party to come back and say that there is some unexpected problem which jeopardises their ability to get it ratified. These things are often settled in side letters which don't appear in the treaty text but which supersede treaty provisions for those parties, so you don't necessarily know about them if you just read the treaty itself. Sometimes the issue never gets settled but if the issue is not fundamental to the treaty itself then both sides may go ahead anyway while nullifying that part.
The treaty affected both federal and provincial levels of government in Canada, as well as on the EU side as well. In Canada this was handled by a consultation process which kept provincial governments in the loop and ensured that their agreement on points which were in their jurisdiction was secured in advance. Consultation on the EU side of things seemed to be an absolute train wreck by comparison, see above for just one example. I don't know whether the EU can approve the Brexit withdrawal agreement on their own, or if they need the approval of the parliaments of each EU member. If the latter, then going by Canadian experience don't be surprised if multiple countries raise objections and demand that something be renegotiated. I'm not making any predictions here, just pointing out a potential problem.
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