If you hit the tab key + backspace key repeatedly in a command block, you'll get "alwaysday"

When I hit the tab key + backspace key repeatedly on the command input screen of the command block, at first I get "?", but later it says "alwaysday" for some reason.
I don't know why it comes up or if this is a bug, and I'm scared, so please let me know as soon as possible.

Steps to Reproduce:

1. Place a command block
2. Open the command block
3. Click on the Command Input box
4. Press the Tab and Backspace keys repeatedly

Observed Results:
"alwaysday" sometimes appears in the Command Input box.

Expected Results:
A question mark should always appear.