Unfortunately it's not possible for me to test this due to achievements only being awarded once per account, and there otherwise being no indication of an achievement being triggered.
After 21w18a we didn't have the issue anymore, however, we have not upgraded the server to the 1.18 snapshots, so if the bug would have returned, I wouldn't know.
The issue was at least resolved before the 1.17.1 release.
This issue has disappeared. In 21w14a, the issue still seemed present. After that the worldgen changed back and it took awhile to 'upgrade' to a newer version. This newer version was 21w18a eventually and this does not have the issue anymore. All users can connect fine again.
Tested this in 1.15.2 as well: It behaves the same way (as expected).
You've also reported this before at MC-190383
As far as I know: this command is for block entities.
You showed a video on Twitter, mentioned here: https://twitter.com/pesegolsson/status/1271144468952035328
I tried repeating this in the latest pre-release in Creative and Survival. In Creative this indeed seems to be happening, but not in survival.
You also mentioned related bugs which I hope you will add to the ticket. This does not seem furnace array related.
Steps to reproduce
Open any world
Set yourself in Creative mode
Throw 27 stacks of items on the ground with Ctrl+Q
Pick up the stacks
The resulting amount is less than the thrown amount. I've tested stacks of Stone and individual swords, both had the same result. I've also testing switching to survival after throwing and before picking up. This results in less items in the inventory as well. So it seems that it's throwing related.
I've ran into this issue today. The title and description of this bugreport is very unclear. I hope the mods can improve this ticket.
Improved title: Saving and re-opening the world respawns the dragon during the first fight
Affected versions: I've confirmed this for 1.15.2, 1.16 Pre-release 2
Video showing this issue: (in 1.16 Pre-release 2) https://www.youtube.com/watch?v=P0syIczLpOU
Steps to reproduce
Have an end without End Gateways (for example, a 'fresh' one the first time you enter the End)
Remove all the end crystals by punching or shooting them
Damage the dragon
Save the world
Re-open the world
The dragon will now despawn and a new full-health dragon will spawn. This also happens if you do not damage the dragon, but the effect is harder to notice then.
I've tried this, and I can do this indeed in creative mode. I could however not reproduce it in survival mode.