I have used SAI for drawing lineart for my picture. Recently I've had a problem, where my progress was undone the next time I opened the file, despite pressing "Save".

Of course if it just happened once, I'd accept it as a clumsy mistake of mine, but it keeps repeating, and I am growing concerned over what's causing it.

Could anyone help?

The software and all the files are stored in my Dropbox directory. I am mentioning this, but I wish to note that in all the years past, I've not had this issue come up.