The issue is present on cawiki and dewiki.
- On Special:RecentChanges (Special:Watchlist) click on the bookmark icon to save a filter.
- The dialog box will be displayed - the bookmark icon is displayed on a separate line (may be it's worth fixing it too).
- Click in the check box "Set as defaul", "Create default filter" button will be pushed below 'Cancel'.
The same issue is present on dewiki: