For mails with lots of single attachments, the graphical display of attachments in a read window might be displayed truncated. This happens in case the attachment list object cannot enlarge itself to the required size (e.g. because the window is to small). Unfortunately it is even not possible to display/use a scrollbar for those cases due to technical reasons.

But as the attachment list is virtual group object you should be able to use the virtual scrolling capabilities of MUI for that case. Please consult the MUI configuration for setting the scroll action hotkey/mouse button. But per default, MUI should allow you to hold down the middle mouse button while moving your mouse. This should then allow you to scroll the attachment list and view the other attachments that are normally hided. Another way for displaying all attachments of a mail is, of course to use the respective toolbar buttons (Display, Print, etc.) which will present you a full list of the attachments.