"Watchlist" (having multiple pages) -> "mark all messages as read" = All topics from 1st page are marked as read, as the url specifies all topics that should be marked as read.
"Show new replies to your posts." (having multiple pages) -> exactly same behaviour as the above.
"Show unread posts since last visit." (having multiple pages) -> "mark all messages as read" = ALL topics are marked as read. (url does not specify which topics should be marked as read).
I'm not sure if this is supposed to work like this, but it's the main reason I prefer "Watchlist" over "Show unread posts since last visit.", nowadays.
Please dont(!) fix that. I was really happy when I once had 10+ page one my watchlist that I could only mark a few very old as read. Took me a while to be brave enough, because I suspected all would be marked as read.
I think you misunderstood what I was asking if it should be changed:
"Watchlist" (having multiple pages) -> "mark all messages as read" = Only topics from 1st page are marked as read, as the url specifies all topics that should be marked as read.
"Show new replies to your posts." (having multiple pages) -> "mark all messages as read" = Only topics from 1st page are marked as read, as the url specifies all topics that should be marked as read.
"Show unread posts since last visit." (having multiple pages) -> "mark all messages as read" = ALL topics are marked as read.
I think it would be more helpful if this "unread" list action worked as in "watchlist" and "unread replies".
Well the link says "all" so I expected the fix to indeed mark "all" as read, on all pages, not just the one shown. Maybe rename it to "mark this page as read" for clarification or add two links. It should certainly be consistant over all pages.