relogging with default client seems to refreshed the tiles. but i assume it took the map tiles from inside my stone tower. can't seem to reproduce easily
I have seen this issue as well -- though less so recently, come to think of it -- and would love to fix it if steps to reproduce can be found, or a hint of what's going on.
"The psychological trials of dwellers in the last times will be equal to the physical trials of the martyrs. In order to face these trials we must be living in a different world."
My theory, without looking into it with a debugger, is that the files are (at least on windows) locked by another thread (from writing) and inaccessible when trying to open them (for reading) from other threads.
Or, in case the opening succeeds (then sharing the handle between the writer and the map reader) the cursor is at the end (from the write that just happened).
Or, similiar to the last scenario, the writer closes the handle before the reader read the data.
Linux most likely behaves differently (more sanely, most times, in my experience).