This is a minor improvement to the multiple start-spawn update in 1.17.
The distribution of players to start spawns at the start of a game in 1.17 was purely random, which meant it was theoretically possible (but unlikely) that everyone could spawn on the same platform.
This update ensures even distribution of players across the available start spawns. If you have 4 start spawns and 8 players you will get exactly 2 players at each start spawn.
There were no known bugs in 1.17 and this is just a minor improvement so, if you have already downloaded 1.17, there's no rush to update to 1.17.1.