Steps to reproduce:
Open the chat, run:

/say

Or run this:

/tell @s

In Bedrock Edition, they run successful (see Attach 1).
In Java Edition, they run fail (see Attach 2).

• Syntax of /say:
  JE:

  /say <message>

  BE:

  /say <message: message>

Observed result:
Runs successful (in BE).

Expected result:
Runs fail (in BE).
Reason: The command is incomplete, or any argument is not specified correctly.