We are aware that unfilled map tiles are too visually similar to filled map tiles, so if that’s the issue, we’re hoping to correct it soon.
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I think that's the hint I needed. 😊 You're right that I couldn't tell that certain tiles were unvisited and accessible. I found two in the upper right, and so there's one left, which is probably in a tile that looks like it's unfilled and therefore accessible as you say. (Columns 5 and 6, row 2.)
Edit: That was it! Thanks!