The Bug:
Similarly to MC-253497, running the command "/execute as player run say a message" you can make another player send a signed message, and it is reportable in the report GUI.
you could make a player send a message that contains things againts the TOS, that they would not send otherwise, and report them. this can be used to ban players for something they did not say, and completely prevent that player from playing.

Steps To Reproduce:

1. join a server with other players as an operator, or a user with the permission to run /execute
2. run the command: /execute as "name of another player" run say "a message that contains malicious content"
3. report the player you made say the message in the report UI.

Expected Behavior:

The message you sent as another player should not be reportable in the GUI.

Observed Behavior:

The message you sent as another player is reportable in the GUI, and gets reported successfully.