The bug

The /teleport doesn't seem to check the number bounds anymore. Compare to 1.12, teleporting past 30 million is not possible and 1e1000 is not considered a valid value.

You can also teleport things to NaN, which also fails to parse as a valid coordinate in 1.12.

How to reproduce

Issue the following commands:

/summon armor_stand
/teleport @e[type=armor_stand] ~ ~ 1e1000

→ Teleported Armor Stand to something, something, Infinity appears in chat. The armor stand does not move.
→ The server freezes (chunkloading doesn't work, chat unresponsive, you know the drill)