It happens sometimes that Spaces are marked as read, although I have not actively read new messages. Often because I am simply still in the Space, although Webex is not actively used by me at that moment (Webex is one of many windows on my screen).
Practical is of course the "mark as unread" option, but it would be even better if Spaces are marked as read only when I have actively done something in Space. For example, when I have written a message, used a reaction ... or when I actively click on a read button in the space. Spaces should not be marked as read just because i have the space open.
Sounds old-fashioned, but this way you don't lose the overview so fast any longer and I decide when a Space is marked as read.