I believe that previously, comments sent to a file were notified on the desktop and in the Slack app, even when the file was open.
However, recently, it seems that notifications are no longer received when the file is kept open. (Apologies if I'm mistaken.)
When a tab for another file is open, notifications appear to be delivered normally.
I often operate other browser apps while keeping the tab for the file I want to receive notifications for open in the desktop app, so in such cases, I often don't notice the notifications even if the file remains open.
I would like you to consider either allowing notifications to be received even when the file is active on the tab, as before, or providing such an option.