The bug

Tab-completion of a command or its arguments does not work in command blocks with command suggestions off. However, the syntax help does still appear in this case. This happens both when the command is and isn't prefixed with /.

How to reproduce

• Turn off "Command Suggestions" in chat settings
• Place a command block
• Type /play and then press tab ⇒ nothing happens
• Replace the text with play and then press tab ⇒ nothing happens
• Replace the text with /playsound minecraft: and then press tab ⇒ nothing happens (however, command syntax for /playsound will be displayed)
• Replace the text with playsound minecraft: and then press tab ⇒ nothing happens (however, command syntax for /playsound will be displayed)
• Try the same process in normal chat (still with command suggestions off) ⇒ it will still provide completions.