I don’t think that this is an intended behavior, hence it is a kind of bug, but somehow I can make use of it for sharing notes in our language within expression editor to my team whiles we learning Samples Apps.
How to reproduce:
Place a back-tick (`) in the expression editor at the end of expression or assign (your_long_expression_is_here) then put any notes - expression will still shown as passed (those notes were not being considered as part of calculation).
Here are some samples, I can even place emoji within expression editor:
Those notes are still remains even after close the apps and reload/reopen, and the calculation/result of the expression is the calculating as if there are no notes or other illegal characters.
This bug were discovered using a free Appsheet - Microsoft account with browser Microsoft Edge. I have not test it for any other environments nor in my other account using google.
Personally, I am not sure if I want this bug to be closed, however even a nice bug is still a bug which may lead to something, unless someone is confirming that it is an intended behavior (say for a future feature deployment) or it is save to be utilized as note taking within expression editor.