It’s not a glitch, you just need to modify your settings a bit. The shortcodes are not aware of each other, so if you stay on the same page when you click a sign-up link, every shortcode you have on the page (8 of them in your case), is going to do its job and generate a sign-up form for the sheet & task id in the URL.
You need to set up one other page with just the main shortcode all by itself (which would list all your sheets in a simple table, and is also used to show user’s which tasks they have signed up for and allow them to clear themselves). Then, in the settings for the sign-up sheets, select the new page (that only has one shortcode on it) in the Volunteer Sign Up Page select box, and save your settings. After that, when somebody clicks on a sign-up link on any of those individual sheets you have on the other page, they will be redirect to the main sign-up page and there will only be one sign-up form (since there is only one shortcode there).