I've been having the same issue ever since "upgrading" to the dark theme. It seems to primarily be happening on one particular note but I've been hesitant to do much editing over the last two weeks. The note I noticed it on is a massive note that I put all of one type of info on, which happens to be my most critical and important note.
I use the app on my MacBook Pro, my iPhone XR, and a windows/google chrome browser at work. I make formatting changes on my primary device (laptop) and when I go to work, log into the website via the google chrome browser on a windows computer, the changes