Thankfully, the next time I played, the map repainted the explored areas.
I've had a map blank on me in 1.6.2. And just like Poker Chen commented, I noticed that the new map I was creating briefly had the same map # (#3) as a previously created map and then switched to a new # (#7). After this, map #3 was blank. 😞
Thankfully, the next time I played, the map repainted the explored areas.