As title says, map was just a black box, but in the overworld it had tried to render a tiny part before stopping. I went to that part of the map, and I saw a beehive nearby. After covering it on top with a dirt block, and restarting my game, the map worked correctly.

The beehive was at honey_level:5, for what it's worth, and beehives pick up a new texture when full, maybe that's relevant?

I need to see logs to say something. May be it some error in the updating thread.

I actually don't know if it was specifically the hive now, it just seems to happen rarely while I'm playing on my friend's 20w13b server; if it happens again I'll try to grab the log for it, but what it does is just keep rendering as normal but rarely and randomly it'll stop drawing new/unknown blocks to the map

it's happened to my friend also, so it's not just me, and thus far it's happened in both overworld and nether
I've switched to the version you released today, so if it never happens again, maybe you fixed it