This has happened to me a few times since I started using Todoist a few weeks ago. I update my list on my mac or on my iPhone. Then later when I access Todoist, the changes are lost. Sometimes they are completely lost I'm back to a couple iterations past version of my to do lists. Other times some newer tasks or changes I've entered are intact and at the same time odd things show up from several days or a week or two past. For example, old deadlines that I changed are back in place. Items I have added are gone. Obviously I need to rely on Todoist to remember changes I enter.
Well, I suspect this could be reproduced by making some changes on macbook and then making other changes on iPhone then accessing mac again and seeing what in in sync. Often the sync works but it's failed enough times that I'm starting to think Todoist won't work for me because I can't trust it to remember what I enter. It doesn't just go back to the version before the most recent changes. It goes back further in some ways and sometimes leaves more recent changes intact.
One idea I have is that perhaps it may have something to do with keeping a window on my mac open with todoist in it and then starting a new one later that day or the next, not recognizing that the other window is open. Perhaps Todoist can't tell what I've entered most recently somehow. Things get muddled. I can't say for sure if this has happened every time, but it could be a factor.