There have a been a few important cases on vandalism on the map in the past weeks, by people drawing long roads across the entire globe or writing abusive comments about the war in Ukraine.  Vandals have continuously abused the "open" nature of OSM by creating tons of new accounts and using bots to flood the map with hundreds of bad edits.

This vandalism has been promptly reverted and new initiatives have been taken to prevent new accounts from doing worldwide damage to OSM.

Since the map on osm.org is made of cached PNG static images (which we call "tiles"), and as the tile servers are extremely busy regenerating every tile on the planet, it can take a few days so that everyone get served a "clean" version of the map.  Each zoom level is a separate collection of tiles, that also explains why the problem is still visible on some zoom levels and not on others.

The note was written 4 days ago.  As of this Friday, it looks like the problem is nowhere to be seen anymore.