Summary: Manually typing in the tilde character (~) for relative coordinates results in the player being unable to affect the game in any way, whether they are composing the command in the chat box or in a command block GUI. For example, beginning to type /tp @s ~ ~1 ~ results in the player often not being able to finish typing the command, followed by the player being unable to exit the chat/command block GUI or affect the game through any inputs.

Expected vs. Result: Intuitively, the player should not be soft-locked while composing a command, and I expected to be able to continue to write the command or exit the chat/command block GUI by means of pressing Escape, Enter, or clicking Done or Cancel in the case of the command block. All of these methods did not work, and the game seemed to ignore these inputs, effectively soft-locking the player. However, the game did not appear to be frozen, as my framerate continued to fluctuated and mobs still moved in the background (if I wasn't in a command block)

Steps to Reproduce:

1. (Optional) Create a new world to ensure no other influencing factors. Ensure the player can run commands in this world
2. Open the chat prompt to being composing a command that can take relative coordinates as arguments (e.g. setblock, tp). For example, typing up to /tp @s ~ ~ (at least in my testing), resulted in any further keyboard inputs failing to register. Similar results occurred for /setblock ~ ~
3. After restarting the game and reopening the world, the player is no longer softlocked. Run /give @s minecraft:command_block to try out the command block GUI.
4. Typing in /setblock ~ ~ will yet again result in the aforementioned softlock

Further Notes:

• I tried to reproduce this bug on my laptop (Windows 10 Home, Intel i7-770HQ @ 2.8 GHz, GTX 1050, same Java edition) and could not replicate this at all.