I also having the same problem. The map doesn't seem to have to refresh for me. I spent a couple hours playing and such, then had a break where I shut down everything. Then when I started everything up again, it generated the same map that I had in the previous
I've tried quiting and opening terraria/moreterra. Logging in and out. Deleting all previous map images.
I'm not sure of where to see a time stamp anywhere. But the previous mining that I did, was unchanged.
Also I was wondering if it was possible to generate a map from a joined server. I just dont see a map file anywhere. Just my single player ones.