Alt key/modifier not transmitted
When using a modal editor (mine is Helix Editor), the Alt key is often used as a modifier. But, for some obscure reasons, sometime the key event is no longer transmitted to the running app. It happens randomly by periods, I was not able to identify any reproducing factors or conditions. It only affects the Alt key.
I did disabled "all menu access key via Alt", but interestingly, the menu bar still get focused when the Alt key is held pressed down (mnemonic character underlining). I also removed all the shortcuts that were using the Alt key.