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 are gone. I know it sync'd because I'm pushing the sync button. I've also checked the sync history. I've logged out of all devices and then back in, but that didn't help either. I'm not 100% sure but it seems like I have to make formatting changes on the windows computer via browser for the changes to stick, which is terrible since that's the only non-dark theme and it has limited color/formatting choices compared to my laptop.
Also, it's a joke that you can't submit an actual tech support ticket without paying for Evernote! A bug is a valid reason to contact support and get the issue resolved. It verges on the ridiculous to pay them $7.99/mo just to have access to tell them they have a bug that needs to be fixed ASAP.