This is definitely a problem, and notes entered when offline also get lost (which is even more of a problem, as they’re not stored somewhere else they can be retrieved from, like photos). It’s been mentioned in this thread, but unfortunately no response from Monzo, so not sure if it’s registered as a bug they will fix. 
Also, I think when I had the problem described in my response on that thread 
, I lost the photo as if taken it directly in Monzo instead of choosing one from the photo library.