The bug
When a malformed command is typed into a command block, then the screen is closed and reopened, the error is displayed in the previous output window. However, if the command is then modified to be correct, the error remains. This is confusing because it makes it look like your command has a syntax error, when it doesn't.
How to reproduce
1. Open a command block
2. Type give
3. Close the menu
4. Reopen it
5. Fix the command by typing give @a dirt
6. Close the menu
7. Reopen it
→ ❌ The previous output section still thinks there's a syntax error after "give"
Notes
If the previous output is showing legitimate command output and not syntax information, it's important that that output is not lost/overridden when viewing the command block and leaving the command unchanged.
Probably the best solution is to show the syntax errors in real-time like Java does but we all know that's not going to happen
I think it works as intended. The message will not update until there is another error to be highlighted or you activate the command.